trait_exists

<?phptrait World {    private static $instance;    protected $tmp;    public static function World()    {        self::$instance = new static();        self::$instance->tmp = get_called_class().' '.__TRAIT__;                return self::$instance;    }}if ( trait_exists( 'World' ) ) {        class Hello {        use World;        public function text( $str )        {            return $this->tmp.$str;        }    }}echo Hello::World()->text('!!!'); // Hello World!!!

Traits are compatible with class autoload mechanism - in fact, if you look at source code of trait_exists function, you will find similar peace of code (see Zend/zend_builtin_functions.c)

What is the default value of $autoload? And in which way traits are autoloaded? Is there something as spl_autoload() for traits?