ma per richiamare la funzione 'autoload' come devo fare?
Hai letto http://php.net/manual/en/language.oop5.autoload.php? E' scritto nelle prime righe,
__autoload() function which is automatically called in case you are trying to use a class/interface which hasn't been defined yet. By calling this function the scripting engine is given a last chance to load the class before PHP fails with an error.