PHP: trait_exists - Manual

<?php
trait 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?