require_once('/path/to/oauth2-server-php/src/OAuth2/Autoloader.php'); OAuth2\Autoloader::register(); Using Composer? Execute the following command: composer.phar require bshaffer/oauth2-server-php "^1.10" This will add the requirement to the composer.json and install the library. It is highly recommended you check out the v1.10.0 tag to ensure your application doesn’t break from backwards-compatib