spl_autoload_register( function ( $class_name ) {
    $models_path = __DIR__ . '/models/' . $class_name . '.php';
    $services_path = __DIR__ . '/services/' . $class_name . '.php';

    if ( file_exists( $models_path) ) {
        require( $models_path );
    } else if ( file_exists( $services_path ) ) {
        require( $services_path );
    }
} );