autoload.php

<?php
    /**
     * A simple PSR-4 autoloader for the tropo-webapi-php library
     *
     * After registering this autoload function with SPL, the following line
     * would cause the function to attempt to load the \Tropo\Foo\Bar class
     * from <current path>/src/Foo/Bar.php:
     *
     *      new \Tropo\Foo\Bar;
     *
     * @param string $class The fully-qualified class name.
     *
     * @return void
     */
    spl_autoload_register(function ($class) {

        // Namespace prefix
        $prefix = 'Tropo\\';

        // Base directory for the namespace prefix
        $base_dir = realpath(__DIR__) . '/src/';

        // Does the requested class use the namespace prefix?
        $len = strlen($prefix);
        if (strncmp($prefix, $class, $len) !== 0) {
            // no, move to the next registered autoloader
            return;
        }

        // Get the relative class name
        $relative_class = substr($class, $len);

        // Replace the namespace prefix with the base directory, replace namespace
        // separators with directory separators in the relative class name, append
        // with .php
        $file = $base_dir . str_replace('\\', '/', $relative_class) . '.php';

        // if the file exists, require it
        if (file_exists($file)) {
            require $file;
        }
    });