trait_exists

<?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?