diff --git a/src/Prometheus/Collector.php b/src/Prometheus/Collector.php
index a63420e..edf9c58 100644
--- a/src/Prometheus/Collector.php
+++ b/src/Prometheus/Collector.php
@@ -9,7 +9,7 @@
 
 abstract class Collector
 {
-    const RE_METRIC_LABEL_NAME = '/^[a-zA-Z_:][a-zA-Z0-9_:]*$/';
+    protected const RE_METRIC_LABEL_NAME = '/^[a-zA-Z_:][a-zA-Z0-9_:]*$/';
 
     /**
      * @var Adapter
@@ -58,7 +58,7 @@ public function __construct(Adapter $storageAdapter, $namespace, $name, $help, $
     /**
      * @return string
      */
-    abstract public function getType();
+    abstract public function getType(): string;
 
     /**
      * @return string
diff --git a/src/Prometheus/Counter.php b/src/Prometheus/Counter.php
index 1b095e5..2cd847d 100644
--- a/src/Prometheus/Counter.php
+++ b/src/Prometheus/Counter.php
@@ -8,7 +8,7 @@
 
 class Counter extends Collector
 {
-    const TYPE = 'counter';
+    public const TYPE = 'counter';
 
     /**
      * @return string
diff --git a/src/Prometheus/Gauge.php b/src/Prometheus/Gauge.php
index 72189f7..4a65144 100644
--- a/src/Prometheus/Gauge.php
+++ b/src/Prometheus/Gauge.php
@@ -8,7 +8,7 @@
 
 class Gauge extends Collector
 {
-    const TYPE = 'gauge';
+    public const TYPE = 'gauge';
 
     /**
      * @param double $value e.g. 123
diff --git a/src/Prometheus/Histogram.php b/src/Prometheus/Histogram.php
index ff1a48f..ab0f13b 100644
--- a/src/Prometheus/Histogram.php
+++ b/src/Prometheus/Histogram.php
@@ -9,7 +9,7 @@
 
 class Histogram extends Collector
 {
-    const TYPE = 'histogram';
+    public const TYPE = 'histogram';
 
     /**
      * @var array|null
diff --git a/src/Prometheus/RenderTextFormat.php b/src/Prometheus/RenderTextFormat.php
index a5b78dc..a598466 100644
--- a/src/Prometheus/RenderTextFormat.php
+++ b/src/Prometheus/RenderTextFormat.php
@@ -6,7 +6,7 @@
 
 class RenderTextFormat
 {
-    const MIME_TYPE = 'text/plain; version=0.0.4';
+    public const MIME_TYPE = 'text/plain; version=0.0.4';
 
     /**
      * @param MetricFamilySamples[] $metrics
diff --git a/src/Prometheus/Sample.php b/src/Prometheus/Sample.php
index c8774e8..d1bfdc6 100644
--- a/src/Prometheus/Sample.php
+++ b/src/Prometheus/Sample.php
@@ -51,7 +51,7 @@ public function getName(): string
      */
     public function getLabelNames(): array
     {
-        return (array)$this->labelNames;
+        return (array) $this->labelNames;
     }
 
     /**
@@ -59,7 +59,7 @@ public function getLabelNames(): array
      */
     public function getLabelValues(): array
     {
-        return (array)$this->labelValues;
+        return (array) $this->labelValues;
     }
 
     /**
diff --git a/src/Prometheus/Storage/APC.php b/src/Prometheus/Storage/APC.php
index 2fe659d..dde3c64 100644
--- a/src/Prometheus/Storage/APC.php
+++ b/src/Prometheus/Storage/APC.php
@@ -10,7 +10,7 @@
 
 class APC implements Adapter
 {
-    const PROMETHEUS_PREFIX = 'prom';
+    private const PROMETHEUS_PREFIX = 'prom';
 
     /**
      * @return MetricFamilySamples[]
@@ -251,7 +251,7 @@ private function collectHistograms(): array
                 $acc = 0;
                 $decodedLabelValues = $this->decodeLabelValues($labelValues);
                 foreach ($data['buckets'] as $bucket) {
-                    $bucket = (string)$bucket;
+                    $bucket = (string) $bucket;
                     if (!isset($histogramBuckets[$labelValues][$bucket])) {
                         $data['samples'][] = [
                             'name' => $metaData['name'] . '_bucket',
diff --git a/src/Prometheus/Storage/Adapter.php b/src/Prometheus/Storage/Adapter.php
index 068e1d9..3bcad49 100644
--- a/src/Prometheus/Storage/Adapter.php
+++ b/src/Prometheus/Storage/Adapter.php
@@ -8,14 +8,14 @@
 
 interface Adapter
 {
-    const COMMAND_INCREMENT_INTEGER = 1;
-    const COMMAND_INCREMENT_FLOAT = 2;
-    const COMMAND_SET = 3;
+    public const COMMAND_INCREMENT_INTEGER = 1;
+    public const COMMAND_INCREMENT_FLOAT = 2;
+    public const COMMAND_SET = 3;
 
     /**
      * @return MetricFamilySamples[]
      */
-    public function collect();
+    public function collect(): array;
 
     /**
      * @param array $data
diff --git a/src/Prometheus/Storage/InMemory.php b/src/Prometheus/Storage/InMemory.php
index bfd4c70..9c59a0d 100644
--- a/src/Prometheus/Storage/InMemory.php
+++ b/src/Prometheus/Storage/InMemory.php
@@ -67,7 +67,7 @@ private function collectHistograms(): array
                 $acc = 0;
                 $decodedLabelValues = $this->decodeLabelValues($labelValues);
                 foreach ($data['buckets'] as $bucket) {
-                    $bucket = (string)$bucket;
+                    $bucket = (string) $bucket;
                     if (!isset($histogramBuckets[$labelValues][$bucket])) {
                         $data['samples'][] = [
                             'name' => $metaData['name'] . '_bucket',
diff --git a/src/Prometheus/Storage/Redis.php b/src/Prometheus/Storage/Redis.php
index 5570685..4b31bcd 100644
--- a/src/Prometheus/Storage/Redis.php
+++ b/src/Prometheus/Storage/Redis.php
@@ -13,7 +13,7 @@
 
 class Redis implements Adapter
 {
-    const PROMETHEUS_METRIC_KEYS_SUFFIX = '_METRIC_KEYS';
+    private const PROMETHEUS_METRIC_KEYS_SUFFIX = '_METRIC_KEYS';
 
     /**
      * @var array
diff --git a/tests/Test/BlackBoxPushGatewayTest.php b/tests/Test/BlackBoxPushGatewayTest.php
index b3ad8ac..6fc33bf 100644
--- a/tests/Test/BlackBoxPushGatewayTest.php
+++ b/tests/Test/BlackBoxPushGatewayTest.php
@@ -13,7 +13,7 @@ class BlackBoxPushGatewayTest extends TestCase
     /**
      * @test
      */
-    public function pushGatewayShouldWork()
+    public function pushGatewayShouldWork(): void
     {
         $adapter = new APC();
         $registry = new CollectorRegistry($adapter);
diff --git a/tests/Test/BlackBoxTest.php b/tests/Test/BlackBoxTest.php
index b4d8a9e..9f43a59 100644
--- a/tests/Test/BlackBoxTest.php
+++ b/tests/Test/BlackBoxTest.php
@@ -18,7 +18,7 @@ class BlackBoxTest extends TestCase
      */
     private $adapter;
 
-    public function setUp()
+    public function setUp(): void
     {
         $this->adapter = getenv('ADAPTER');
         $this->client = new Client(['base_uri' => 'http://nginx:80/']);
@@ -28,7 +28,7 @@ public function setUp()
     /**
      * @test
      */
-    public function gaugesShouldBeOverwritten()
+    public function gaugesShouldBeOverwritten(): void
     {
         $start = microtime(true);
         $promises = [
@@ -43,7 +43,7 @@ public function gaugesShouldBeOverwritten()
         echo "\ntime: " . ($end - $start) . "\n";
 
         $metricsResult = $this->client->get('/examples/metrics.php?adapter=' . $this->adapter);
-        $body = (string)$metricsResult->getBody();
+        $body = (string) $metricsResult->getBody();
         echo "\nbody: " . $body . "\n";
         $this->assertThat(
             $body,
@@ -58,13 +58,13 @@ public function gaugesShouldBeOverwritten()
     /**
      * @test
      */
-    public function countersShouldIncrementAtomically()
+    public function countersShouldIncrementAtomically(): void
     {
         $start = microtime(true);
         $promises = [];
         $sum = 0;
         for ($i = 0; $i < 1100; $i++) {
-            $promises[] =  $this->client->getAsync('/examples/some_counter.php?c=' . $i . '&adapter=' . $this->adapter);
+            $promises[] = $this->client->getAsync('/examples/some_counter.php?c=' . $i . '&adapter=' . $this->adapter);
             $sum += $i;
         }
 
@@ -73,7 +73,7 @@ public function countersShouldIncrementAtomically()
         echo "\ntime: " . ($end - $start) . "\n";
 
         $metricsResult = $this->client->get('/examples/metrics.php?adapter=' . $this->adapter);
-        $body = (string)$metricsResult->getBody();
+        $body = (string) $metricsResult->getBody();
 
         $this->assertThat($body, $this->stringContains('test_some_counter{type="blue"} ' . $sum));
     }
@@ -81,7 +81,7 @@ public function countersShouldIncrementAtomically()
     /**
      * @test
      */
-    public function histogramsShouldIncrementAtomically()
+    public function histogramsShouldIncrementAtomically(): void
     {
         $start = microtime(true);
         $promises = [
@@ -102,7 +102,7 @@ public function histogramsShouldIncrementAtomically()
         echo "\ntime: " . ($end - $start) . "\n";
 
         $metricsResult = $this->client->get('/examples/metrics.php?adapter=' . $this->adapter);
-        $body = (string)$metricsResult->getBody();
+        $body = (string) $metricsResult->getBody();
 
         $this->assertThat($body, $this->stringContains(<<<EOF
 test_some_histogram_bucket{type="blue",le="0.1"} 1
diff --git a/tests/Test/Prometheus/APC/CollectorRegistryTest.php b/tests/Test/Prometheus/APC/CollectorRegistryTest.php
index 9d73fb7..1196084 100644
--- a/tests/Test/Prometheus/APC/CollectorRegistryTest.php
+++ b/tests/Test/Prometheus/APC/CollectorRegistryTest.php
@@ -10,8 +10,7 @@
  */
 class CollectorRegistryTest extends AbstractCollectorRegistryTest
 {
-
-    public function configureAdapter()
+    public function configureAdapter(): void
     {
         $this->adapter = new APC();
         $this->adapter->flushAPC();
diff --git a/tests/Test/Prometheus/APC/CounterTest.php b/tests/Test/Prometheus/APC/CounterTest.php
index 306ec36..9b3421c 100644
--- a/tests/Test/Prometheus/APC/CounterTest.php
+++ b/tests/Test/Prometheus/APC/CounterTest.php
@@ -11,7 +11,7 @@
  */
 class CounterTest extends AbstractCounterTest
 {
-    public function configureAdapter()
+    public function configureAdapter(): void
     {
         $this->adapter = new APC();
         $this->adapter->flushAPC();
diff --git a/tests/Test/Prometheus/APC/GaugeTest.php b/tests/Test/Prometheus/APC/GaugeTest.php
index c5dc4f0..b1da8c3 100644
--- a/tests/Test/Prometheus/APC/GaugeTest.php
+++ b/tests/Test/Prometheus/APC/GaugeTest.php
@@ -11,7 +11,7 @@
  */
 class GaugeTest extends AbstractGaugeTest
 {
-    public function configureAdapter()
+    public function configureAdapter(): void
     {
         $this->adapter = new APC();
         $this->adapter->flushAPC();
diff --git a/tests/Test/Prometheus/APC/HistogramTest.php b/tests/Test/Prometheus/APC/HistogramTest.php
index 7ef2bed..777160d 100644
--- a/tests/Test/Prometheus/APC/HistogramTest.php
+++ b/tests/Test/Prometheus/APC/HistogramTest.php
@@ -11,7 +11,7 @@
  */
 class HistogramTest extends AbstractHistogramTest
 {
-    public function configureAdapter()
+    public function configureAdapter(): void
     {
         $this->adapter = new APC();
         $this->adapter->flushAPC();
diff --git a/tests/Test/Prometheus/AbstractCollectorRegistryTest.php b/tests/Test/Prometheus/AbstractCollectorRegistryTest.php
index 76f3160..c0664cb 100644
--- a/tests/Test/Prometheus/AbstractCollectorRegistryTest.php
+++ b/tests/Test/Prometheus/AbstractCollectorRegistryTest.php
@@ -23,6 +23,8 @@ abstract class AbstractCollectorRegistryTest extends TestCase
      */
     private $renderer;
 
+    abstract public function configureAdapter(): void;
+
     public function setUp(): void
     {
         $this->configureAdapter();
@@ -32,7 +34,7 @@ public function setUp(): void
     /**
      * @test
      */
-    public function itShouldSaveGauges()
+    public function itShouldSaveGauges(): void
     {
         $registry = new CollectorRegistry($this->adapter);
 
@@ -63,7 +65,7 @@ public function itShouldSaveGauges()
     /**
      * @test
      */
-    public function itShouldSaveCounters()
+    public function itShouldSaveCounters(): void
     {
         $registry = new CollectorRegistry($this->adapter);
         $metric = $registry->registerCounter('test', 'some_metric', 'this is for testing', ['foo', 'bar']);
@@ -89,7 +91,7 @@ public function itShouldSaveCounters()
     /**
      * @test
      */
-    public function itShouldSaveHistograms()
+    public function itShouldSaveHistograms(): void
     {
         $registry = new CollectorRegistry($this->adapter);
         $metric = $registry->registerHistogram(
@@ -142,7 +144,7 @@ public function itShouldSaveHistograms()
     /**
      * @test
      */
-    public function itShouldSaveHistogramsWithoutLabels()
+    public function itShouldSaveHistogramsWithoutLabels(): void
     {
         $registry = new CollectorRegistry($this->adapter);
         $metric = $registry->registerHistogram('test', 'some_metric', 'this is for testing');
@@ -183,7 +185,7 @@ public function itShouldSaveHistogramsWithoutLabels()
     /**
      * @test
      */
-    public function itShouldIncreaseACounterWithoutNamespace()
+    public function itShouldIncreaseACounterWithoutNamespace(): void
     {
         $registry = new CollectorRegistry($this->adapter);
         $registry
@@ -207,7 +209,7 @@ public function itShouldIncreaseACounterWithoutNamespace()
     /**
      * @test
      */
-    public function itShouldForbidRegisteringTheSameCounterTwice()
+    public function itShouldForbidRegisteringTheSameCounterTwice(): void
     {
         $registry = new CollectorRegistry($this->adapter);
         $registry->registerCounter('foo', 'metric', 'help');
@@ -219,7 +221,7 @@ public function itShouldForbidRegisteringTheSameCounterTwice()
     /**
      * @test
      */
-    public function itShouldForbidRegisteringTheSameCounterWithDifferentLabels()
+    public function itShouldForbidRegisteringTheSameCounterWithDifferentLabels(): void
     {
         $registry = new CollectorRegistry($this->adapter);
         $registry->registerCounter('foo', 'metric', 'help', ["foo", "bar"]);
@@ -231,7 +233,7 @@ public function itShouldForbidRegisteringTheSameCounterWithDifferentLabels()
     /**
      * @test
      */
-    public function itShouldForbidRegisteringTheSameHistogramTwice()
+    public function itShouldForbidRegisteringTheSameHistogramTwice(): void
     {
         $registry = new CollectorRegistry($this->adapter);
         $registry->registerHistogram('foo', 'metric', 'help');
@@ -243,7 +245,7 @@ public function itShouldForbidRegisteringTheSameHistogramTwice()
     /**
      * @test
      */
-    public function itShouldForbidRegisteringTheSameHistogramWithDifferentLabels()
+    public function itShouldForbidRegisteringTheSameHistogramWithDifferentLabels(): void
     {
         $registry = new CollectorRegistry($this->adapter);
         $registry->registerCounter('foo', 'metric', 'help', ["foo", "bar"]);
@@ -255,7 +257,7 @@ public function itShouldForbidRegisteringTheSameHistogramWithDifferentLabels()
     /**
      * @test
      */
-    public function itShouldForbidRegisteringTheSameGaugeTwice()
+    public function itShouldForbidRegisteringTheSameGaugeTwice(): void
     {
         $registry = new CollectorRegistry($this->adapter);
         $registry->registerGauge('foo', 'metric', 'help');
@@ -267,7 +269,7 @@ public function itShouldForbidRegisteringTheSameGaugeTwice()
     /**
      * @test
      */
-    public function itShouldForbidRegisteringTheSameGaugeWithDifferentLabels()
+    public function itShouldForbidRegisteringTheSameGaugeWithDifferentLabels(): void
     {
         $registry = new CollectorRegistry($this->adapter);
         $registry->registerGauge('foo', 'metric', 'help', ["foo", "bar"]);
@@ -279,7 +281,7 @@ public function itShouldForbidRegisteringTheSameGaugeWithDifferentLabels()
     /**
      * @test
      */
-    public function itShouldThrowAnExceptionWhenGettingANonExistentMetric()
+    public function itShouldThrowAnExceptionWhenGettingANonExistentMetric(): void
     {
         $registry = new CollectorRegistry($this->adapter);
 
@@ -290,7 +292,7 @@ public function itShouldThrowAnExceptionWhenGettingANonExistentMetric()
     /**
      * @test
      */
-    public function itShouldNotRegisterACounterTwice()
+    public function itShouldNotRegisterACounterTwice(): void
     {
         $registry = new CollectorRegistry($this->adapter);
         $counterA = $registry->getOrRegisterCounter("foo", "bar", "Help text");
@@ -302,7 +304,7 @@ public function itShouldNotRegisterACounterTwice()
     /**
      * @test
      */
-    public function itShouldNotRegisterAGaugeTwice()
+    public function itShouldNotRegisterAGaugeTwice(): void
     {
         $registry = new CollectorRegistry($this->adapter);
         $gaugeA = $registry->getOrRegisterGauge("foo", "bar", "Help text");
@@ -314,7 +316,7 @@ public function itShouldNotRegisterAGaugeTwice()
     /**
      * @test
      */
-    public function itShouldNotRegisterAHistogramTwice()
+    public function itShouldNotRegisterAHistogramTwice(): void
     {
         $registry = new CollectorRegistry($this->adapter);
         $histogramA = $registry->getOrRegisterHistogram("foo", "bar", "Help text");
@@ -322,7 +324,4 @@ public function itShouldNotRegisterAHistogramTwice()
 
         $this->assertSame($histogramA, $histogramB);
     }
-
-
-    abstract public function configureAdapter();
 }
diff --git a/tests/Test/Prometheus/AbstractCounterTest.php b/tests/Test/Prometheus/AbstractCounterTest.php
index a932939..1073a8f 100644
--- a/tests/Test/Prometheus/AbstractCounterTest.php
+++ b/tests/Test/Prometheus/AbstractCounterTest.php
@@ -19,17 +19,17 @@ abstract class AbstractCounterTest extends TestCase
      */
     public $adapter;
 
+    abstract public function configureAdapter(): void;
+
     public function setUp(): void
     {
         $this->configureAdapter();
     }
 
-    abstract public function configureAdapter();
-
     /**
      * @test
      */
-    public function itShouldIncreaseWithLabels()
+    public function itShouldIncreaseWithLabels(): void
     {
         $counter = new Counter($this->adapter, 'test', 'some_metric', 'this is for testing', ['foo', 'bar']);
         $counter->inc(['lalal', 'lululu']);
@@ -63,7 +63,7 @@ public function itShouldIncreaseWithLabels()
     /**
      * @test
      */
-    public function itShouldIncreaseWithoutLabelWhenNoLabelsAreDefined()
+    public function itShouldIncreaseWithoutLabelWhenNoLabelsAreDefined(): void
     {
         $counter = new Counter($this->adapter, 'test', 'some_metric', 'this is for testing');
         $counter->inc();
@@ -95,7 +95,7 @@ public function itShouldIncreaseWithoutLabelWhenNoLabelsAreDefined()
     /**
      * @test
      */
-    public function itShouldIncreaseTheCounterByAnArbitraryInteger()
+    public function itShouldIncreaseTheCounterByAnArbitraryInteger(): void
     {
         $counter = new Counter($this->adapter, 'test', 'some_metric', 'this is for testing', ['foo', 'bar']);
         $counter->inc(['lalal', 'lululu']);
@@ -128,7 +128,7 @@ public function itShouldIncreaseTheCounterByAnArbitraryInteger()
     /**
      * @test
      */
-    public function itShouldRejectInvalidMetricsNames()
+    public function itShouldRejectInvalidMetricsNames(): void
     {
         $this->expectException(InvalidArgumentException::class);
         new Counter($this->adapter, 'test', 'some metric invalid metric', 'help');
@@ -137,7 +137,7 @@ public function itShouldRejectInvalidMetricsNames()
     /**
      * @test
      */
-    public function itShouldRejectInvalidLabelNames()
+    public function itShouldRejectInvalidLabelNames(): void
     {
         $this->expectException(InvalidArgumentException::class);
         new Counter($this->adapter, 'test', 'some_metric', 'help', ['invalid label']);
@@ -149,7 +149,7 @@ public function itShouldRejectInvalidLabelNames()
      *
      * @param mixed $value The label value
      */
-    public function isShouldAcceptAnySequenceOfBasicLatinCharactersForLabelValues($value)
+    public function isShouldAcceptAnySequenceOfBasicLatinCharactersForLabelValues($value): void
     {
         $label = 'foo';
         $histogram = new Counter($this->adapter, 'test', 'some_metric', 'help', [$label]);
@@ -177,7 +177,7 @@ public function isShouldAcceptAnySequenceOfBasicLatinCharactersForLabelValues($v
      * @return array
      * @see isShouldAcceptArbitraryLabelValues
      */
-    public function labelValuesDataProvider()
+    public function labelValuesDataProvider(): array
     {
         $cases = [];
         // Basic Latin
diff --git a/tests/Test/Prometheus/AbstractGaugeTest.php b/tests/Test/Prometheus/AbstractGaugeTest.php
index 393c087..138e772 100644
--- a/tests/Test/Prometheus/AbstractGaugeTest.php
+++ b/tests/Test/Prometheus/AbstractGaugeTest.php
@@ -19,17 +19,17 @@ abstract class AbstractGaugeTest extends TestCase
      */
     public $adapter;
 
+    abstract public function configureAdapter(): void;
+
     public function setUp(): void
     {
         $this->configureAdapter();
     }
 
-    abstract public function configureAdapter();
-
     /**
      * @test
      */
-    public function itShouldAllowSetWithLabels()
+    public function itShouldAllowSetWithLabels(): void
     {
         $gauge = new Gauge($this->adapter, 'test', 'some_metric', 'this is for testing', ['foo', 'bar']);
         $gauge->set(123, ['lalal', 'lululu']);
@@ -63,7 +63,7 @@ public function itShouldAllowSetWithLabels()
     /**
      * @test
      */
-    public function itShouldAllowSetWithoutLabelWhenNoLabelsAreDefined()
+    public function itShouldAllowSetWithoutLabelWhenNoLabelsAreDefined(): void
     {
         $gauge = new Gauge($this->adapter, 'test', 'some_metric', 'this is for testing');
         $gauge->set(123);
@@ -97,7 +97,7 @@ public function itShouldAllowSetWithoutLabelWhenNoLabelsAreDefined()
     /**
      * @test
      */
-    public function itShouldAllowSetWithAFloatValue()
+    public function itShouldAllowSetWithAFloatValue(): void
     {
         $gauge = new Gauge($this->adapter, 'test', 'some_metric', 'this is for testing');
         $gauge->set(123.5);
@@ -131,7 +131,7 @@ public function itShouldAllowSetWithAFloatValue()
     /**
      * @test
      */
-    public function itShouldIncrementAValue()
+    public function itShouldIncrementAValue(): void
     {
         $gauge = new Gauge($this->adapter, 'test', 'some_metric', 'this is for testing', ['foo', 'bar']);
         $gauge->inc(['lalal', 'lululu']);
@@ -164,7 +164,7 @@ public function itShouldIncrementAValue()
     /**
      * @test
      */
-    public function itShouldIncrementWithFloatValue()
+    public function itShouldIncrementWithFloatValue(): void
     {
         $gauge = new Gauge($this->adapter, 'test', 'some_metric', 'this is for testing', ['foo', 'bar']);
         $gauge->inc(['lalal', 'lululu']);
@@ -197,7 +197,7 @@ public function itShouldIncrementWithFloatValue()
     /**
      * @test
      */
-    public function itShouldDecrementAValue()
+    public function itShouldDecrementAValue(): void
     {
         $gauge = new Gauge($this->adapter, 'test', 'some_metric', 'this is for testing', ['foo', 'bar']);
         $gauge->dec(['lalal', 'lululu']);
@@ -230,7 +230,7 @@ public function itShouldDecrementAValue()
     /**
      * @test
      */
-    public function itShouldDecrementWithFloatValue()
+    public function itShouldDecrementWithFloatValue(): void
     {
         $gauge = new Gauge($this->adapter, 'test', 'some_metric', 'this is for testing', ['foo', 'bar']);
         $gauge->dec(['lalal', 'lululu']);
@@ -263,7 +263,7 @@ public function itShouldDecrementWithFloatValue()
     /**
      * @test
      */
-    public function itShouldOverwriteWhenSettingTwice()
+    public function itShouldOverwriteWhenSettingTwice(): void
     {
         $gauge = new Gauge($this->adapter, 'test', 'some_metric', 'this is for testing', ['foo', 'bar']);
         $gauge->set(123, ['lalal', 'lululu']);
@@ -296,7 +296,7 @@ public function itShouldOverwriteWhenSettingTwice()
     /**
      * @test
      */
-    public function itShouldRejectInvalidMetricsNames()
+    public function itShouldRejectInvalidMetricsNames(): void
     {
         $this->expectException(InvalidArgumentException::class);
         new Gauge($this->adapter, 'test', 'some metric invalid metric', 'help');
@@ -305,7 +305,7 @@ public function itShouldRejectInvalidMetricsNames()
     /**
      * @test
      */
-    public function itShouldRejectInvalidLabelNames()
+    public function itShouldRejectInvalidLabelNames(): void
     {
         $this->expectException(InvalidArgumentException::class);
         new Gauge($this->adapter, 'test', 'some_metric', 'help', ['invalid label']);
@@ -317,7 +317,7 @@ public function itShouldRejectInvalidLabelNames()
      *
      * @param mixed $value The label value
      */
-    public function isShouldAcceptAnySequenceOfBasicLatinCharactersForLabelValues($value)
+    public function isShouldAcceptAnySequenceOfBasicLatinCharactersForLabelValues($value): void
     {
         $label = 'foo';
         $histogram = new Gauge($this->adapter, 'test', 'some_metric', 'help', [$label]);
@@ -345,7 +345,7 @@ public function isShouldAcceptAnySequenceOfBasicLatinCharactersForLabelValues($v
      * @return array
      * @see isShouldAcceptArbitraryLabelValues
      */
-    public function labelValuesDataProvider()
+    public function labelValuesDataProvider(): array
     {
         $cases = [];
         // Basic Latin
diff --git a/tests/Test/Prometheus/AbstractHistogramTest.php b/tests/Test/Prometheus/AbstractHistogramTest.php
index d5f9ebb..324849f 100644
--- a/tests/Test/Prometheus/AbstractHistogramTest.php
+++ b/tests/Test/Prometheus/AbstractHistogramTest.php
@@ -19,17 +19,17 @@ abstract class AbstractHistogramTest extends TestCase
      */
     public $adapter;
 
+    abstract public function configureAdapter(): void;
+
     public function setUp(): void
     {
         $this->configureAdapter();
     }
 
-    abstract public function configureAdapter();
-
     /**
      * @test
      */
-    public function itShouldObserveWithLabels()
+    public function itShouldObserveWithLabels(): void
     {
         $histogram = new Histogram(
             $this->adapter,
@@ -99,7 +99,7 @@ public function itShouldObserveWithLabels()
     /**
      * @test
      */
-    public function itShouldObserveWithoutLabelWhenNoLabelsAreDefined()
+    public function itShouldObserveWithoutLabelWhenNoLabelsAreDefined(): void
     {
         $histogram = new Histogram(
             $this->adapter,
@@ -168,7 +168,7 @@ public function itShouldObserveWithoutLabelWhenNoLabelsAreDefined()
     /**
      * @test
      */
-    public function itShouldObserveValuesOfTypeDouble()
+    public function itShouldObserveValuesOfTypeDouble(): void
     {
         $histogram = new Histogram(
             $this->adapter,
@@ -238,7 +238,7 @@ public function itShouldObserveValuesOfTypeDouble()
     /**
      * @test
      */
-    public function itShouldProvideDefaultBuckets()
+    public function itShouldProvideDefaultBuckets(): void
     {
         // .005, .01, .025, .05, .075, .1, .25, .5, .75, 1.0, 2.5, 5.0, 7.5, 10.0
 
@@ -375,7 +375,7 @@ public function itShouldProvideDefaultBuckets()
     /**
      * @test
      */
-    public function itShouldThrowAnExceptionWhenTheBucketSizesAreNotIncreasing()
+    public function itShouldThrowAnExceptionWhenTheBucketSizesAreNotIncreasing(): void
     {
         $this->expectException(InvalidArgumentException::class);
         $this->expectExceptionMessage('Histogram buckets must be in increasing order');
@@ -385,7 +385,7 @@ public function itShouldThrowAnExceptionWhenTheBucketSizesAreNotIncreasing()
     /**
      * @test
      */
-    public function itShouldThrowAnExceptionWhenThereIsLessThanOneBucket()
+    public function itShouldThrowAnExceptionWhenThereIsLessThanOneBucket(): void
     {
         $this->expectException(InvalidArgumentException::class);
         $this->expectExceptionMessage('Histogram must have at least one bucket');
@@ -395,7 +395,7 @@ public function itShouldThrowAnExceptionWhenThereIsLessThanOneBucket()
     /**
      * @test
      */
-    public function itShouldThrowAnExceptionWhenThereIsALabelNamedLe()
+    public function itShouldThrowAnExceptionWhenThereIsALabelNamedLe(): void
     {
         $this->expectException(InvalidArgumentException::class);
         $this->expectExceptionMessage('Histogram cannot have a label named');
@@ -405,7 +405,7 @@ public function itShouldThrowAnExceptionWhenThereIsALabelNamedLe()
     /**
      * @test
      */
-    public function itShouldRejectInvalidMetricsNames()
+    public function itShouldRejectInvalidMetricsNames(): void
     {
         $this->expectException(InvalidArgumentException::class);
         $this->expectExceptionMessage('Invalid metric name');
@@ -415,7 +415,7 @@ public function itShouldRejectInvalidMetricsNames()
     /**
      * @test
      */
-    public function itShouldRejectInvalidLabelNames()
+    public function itShouldRejectInvalidLabelNames(): void
     {
         $this->expectException(InvalidArgumentException::class);
         $this->expectExceptionMessage('Invalid label name');
@@ -428,7 +428,7 @@ public function itShouldRejectInvalidLabelNames()
      *
      * @param mixed $value The label value
      */
-    public function isShouldAcceptAnySequenceOfBasicLatinCharactersForLabelValues($value)
+    public function isShouldAcceptAnySequenceOfBasicLatinCharactersForLabelValues($value): void
     {
         $label = 'foo';
         $histogram = new Histogram($this->adapter, 'test', 'some_metric', 'help', [$label], [1]);
@@ -455,7 +455,7 @@ public function isShouldAcceptAnySequenceOfBasicLatinCharactersForLabelValues($v
     /**
      * @test
      */
-    public function itShouldBeAbleToGenerateExponentialBucketsGivenSpecificBounds()
+    public function itShouldBeAbleToGenerateExponentialBucketsGivenSpecificBounds(): void
     {
         $start = 0.05;
         $growthFactor = 1.5;
@@ -487,7 +487,7 @@ public function itShouldBeAbleToGenerateExponentialBucketsGivenSpecificBounds()
      * @return array
      * @see isShouldAcceptArbitraryLabelValues
      */
-    public function labelValuesDataProvider()
+    public function labelValuesDataProvider(): array
     {
         $cases = [];
         // Basic Latin
diff --git a/tests/Test/Prometheus/InMemory/CollectorRegistryTest.php b/tests/Test/Prometheus/InMemory/CollectorRegistryTest.php
index 5e6b9c7..aea9b42 100644
--- a/tests/Test/Prometheus/InMemory/CollectorRegistryTest.php
+++ b/tests/Test/Prometheus/InMemory/CollectorRegistryTest.php
@@ -7,7 +7,7 @@
 
 class CollectorRegistryTest extends AbstractCollectorRegistryTest
 {
-    public function configureAdapter()
+    public function configureAdapter(): void
     {
         $this->adapter = new InMemory();
         $this->adapter->flushMemory();
diff --git a/tests/Test/Prometheus/InMemory/CounterTest.php b/tests/Test/Prometheus/InMemory/CounterTest.php
index b2176d8..9f47b7a 100644
--- a/tests/Test/Prometheus/InMemory/CounterTest.php
+++ b/tests/Test/Prometheus/InMemory/CounterTest.php
@@ -10,8 +10,7 @@
  */
 class CounterTest extends AbstractCounterTest
 {
-
-    public function configureAdapter()
+    public function configureAdapter(): void
     {
         $this->adapter = new InMemory();
         $this->adapter->flushMemory();
diff --git a/tests/Test/Prometheus/InMemory/GaugeTest.php b/tests/Test/Prometheus/InMemory/GaugeTest.php
index 84210b6..8397868 100644
--- a/tests/Test/Prometheus/InMemory/GaugeTest.php
+++ b/tests/Test/Prometheus/InMemory/GaugeTest.php
@@ -10,8 +10,7 @@
  */
 class GaugeTest extends AbstractGaugeTest
 {
-
-    public function configureAdapter()
+    public function configureAdapter(): void
     {
         $this->adapter = new InMemory();
         $this->adapter->flushMemory();
diff --git a/tests/Test/Prometheus/InMemory/HistogramTest.php b/tests/Test/Prometheus/InMemory/HistogramTest.php
index 50d402d..8e3cff5 100644
--- a/tests/Test/Prometheus/InMemory/HistogramTest.php
+++ b/tests/Test/Prometheus/InMemory/HistogramTest.php
@@ -10,8 +10,7 @@
  */
 class HistogramTest extends AbstractHistogramTest
 {
-
-    public function configureAdapter()
+    public function configureAdapter(): void
     {
         $this->adapter = new InMemory();
         $this->adapter->flushMemory();
diff --git a/tests/Test/Prometheus/Redis/CollectorRegistryTest.php b/tests/Test/Prometheus/Redis/CollectorRegistryTest.php
index fbf53b9..b2b0899 100644
--- a/tests/Test/Prometheus/Redis/CollectorRegistryTest.php
+++ b/tests/Test/Prometheus/Redis/CollectorRegistryTest.php
@@ -10,7 +10,7 @@
  */
 class CollectorRegistryTest extends AbstractCollectorRegistryTest
 {
-    public function configureAdapter()
+    public function configureAdapter(): void
     {
         $this->adapter = new Redis(['host' => REDIS_HOST]);
         $this->adapter->flushRedis();
diff --git a/tests/Test/Prometheus/Redis/CounterTest.php b/tests/Test/Prometheus/Redis/CounterTest.php
index 4aea278..cf858eb 100644
--- a/tests/Test/Prometheus/Redis/CounterTest.php
+++ b/tests/Test/Prometheus/Redis/CounterTest.php
@@ -11,7 +11,7 @@
  */
 class CounterTest extends AbstractCounterTest
 {
-    public function configureAdapter()
+    public function configureAdapter(): void
     {
         $this->adapter = new Redis(['host' => REDIS_HOST]);
         $this->adapter->flushRedis();
diff --git a/tests/Test/Prometheus/Redis/CounterWithPrefixTest.php b/tests/Test/Prometheus/Redis/CounterWithPrefixTest.php
index 0409140..b578049 100644
--- a/tests/Test/Prometheus/Redis/CounterWithPrefixTest.php
+++ b/tests/Test/Prometheus/Redis/CounterWithPrefixTest.php
@@ -11,7 +11,7 @@
  */
 class CounterWithPrefixTest extends AbstractCounterTest
 {
-    public function configureAdapter()
+    public function configureAdapter(): void
     {
         $connection = new \Redis();
         $connection->connect(REDIS_HOST);
diff --git a/tests/Test/Prometheus/Redis/GaugeTest.php b/tests/Test/Prometheus/Redis/GaugeTest.php
index 6ffd300..c5dfbe6 100644
--- a/tests/Test/Prometheus/Redis/GaugeTest.php
+++ b/tests/Test/Prometheus/Redis/GaugeTest.php
@@ -11,7 +11,7 @@
  */
 class GaugeTest extends AbstractGaugeTest
 {
-    public function configureAdapter()
+    public function configureAdapter(): void
     {
         $this->adapter = new Redis(['host' => REDIS_HOST]);
         $this->adapter->flushRedis();
diff --git a/tests/Test/Prometheus/Redis/GaugeWithPrefixTest.php b/tests/Test/Prometheus/Redis/GaugeWithPrefixTest.php
index 1ba4d0e..4a33d68 100644
--- a/tests/Test/Prometheus/Redis/GaugeWithPrefixTest.php
+++ b/tests/Test/Prometheus/Redis/GaugeWithPrefixTest.php
@@ -11,7 +11,7 @@
  */
 class GaugeWithPrefixTest extends AbstractGaugeTest
 {
-    public function configureAdapter()
+    public function configureAdapter(): void
     {
         $connection = new \Redis();
         $connection->connect(REDIS_HOST);
diff --git a/tests/Test/Prometheus/Redis/HistogramTest.php b/tests/Test/Prometheus/Redis/HistogramTest.php
index 40fd314..3939853 100644
--- a/tests/Test/Prometheus/Redis/HistogramTest.php
+++ b/tests/Test/Prometheus/Redis/HistogramTest.php
@@ -11,7 +11,7 @@
  */
 class HistogramTest extends AbstractHistogramTest
 {
-    public function configureAdapter()
+    public function configureAdapter(): void
     {
         $this->adapter = new Redis(['host' => REDIS_HOST]);
         $this->adapter->flushRedis();
diff --git a/tests/Test/Prometheus/Redis/HistogramWithPrefixTest.php b/tests/Test/Prometheus/Redis/HistogramWithPrefixTest.php
index 4c25388..dc2bded 100644
--- a/tests/Test/Prometheus/Redis/HistogramWithPrefixTest.php
+++ b/tests/Test/Prometheus/Redis/HistogramWithPrefixTest.php
@@ -11,7 +11,7 @@
  */
 class HistogramWithPrefixTest extends AbstractHistogramTest
 {
-    public function configureAdapter()
+    public function configureAdapter(): void
     {
         $connection = new \Redis();
         $connection->connect(REDIS_HOST);
diff --git a/tests/Test/Prometheus/Storage/RedisTest.php b/tests/Test/Prometheus/Storage/RedisTest.php
index 9f2df78..fdf842e 100644
--- a/tests/Test/Prometheus/Storage/RedisTest.php
+++ b/tests/Test/Prometheus/Storage/RedisTest.php
@@ -13,7 +13,7 @@ class RedisTest extends TestCase
     /**
      * @test
      */
-    public function itShouldThrowAnExceptionOnConnectionFailure()
+    public function itShouldThrowAnExceptionOnConnectionFailure(): void
     {
         $redis = new Redis(['host' => '/dev/null']);
 
@@ -27,7 +27,7 @@ public function itShouldThrowAnExceptionOnConnectionFailure()
     /**
      * @test
      */
-    public function itShouldThrowExceptionWhenInjectedRedisIsNotConnected()
+    public function itShouldThrowExceptionWhenInjectedRedisIsNotConnected(): void
     {
         $connection = new \Redis();