PSR-4 autoloader example

autoloader.php

<?php 
/**
* An example of a project-specific implementation.
*
* After registering this autoload function with SPL, the following line
* would cause the function to attempt to load the \Foo\Bar\Baz\Qux class
* from /path/to/project/src/Baz/Qux.php:
*
*
* new \Foo\Bar\Baz\Qux;
*
* @param string $class The fully qualified class name.
* @return void
*/

spl_autoload_register(function ($class) { 
// project-specific namespace prefix
$prefix = 'Foo\\Bar\\';
// base directory for the namespace prefix
$base_dir = __DIR__ . '/src/';
// does the 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;
}
});