This is what I believe to be an implementation of a PSR-4 compatible autoloader. However, I don't have the expertise to guarantee that it is indeed fully PSR-4 compliant. I created this to learn more about Autoloading, and I've used it in some of my own projects. Use it at your own risk!
##Example
//Create a new autoloader instance
$autoloader = new Autoloader();
//Add a single prefix => directory mapping
$autoloader->addPrefix("Abc\\Def", __DIR__ . "/classes/Abc/Def");
//Add multiple mappings at once
$autoloader->addPrefixes("Abc\\Def", [
__DIR__ . "/classes/Abc/Def",
__DIR__ . "otherLocation/Abc/Def"
]);
//Register the autoloader with SPL
$autoloader->register();
//It will now find your classes!
use Abc\Def\Ghi;
$ghi = new Ghi;