if($object != null && is_object($object)){

<?
class MyClass{

public function hello(){
MyClass::world($this);
}

public function ok(){
echo 'ok: ok';
}

static function world($object = null){
if($object != null && is_object($object)){
$object->ok();
echo'World';
}
}

}

$test = new MyClass;
$test->hello();