Consider this PHP code:

Consider this PHP code:

class X {
  public function foo() {
    return "X";
  }
}
class Y extends X {
  public function foo() {
    return "Y";
  }
}
class Z extends Y {
  public function foo() {
    return parent::foo() . parent::foo();
  }
}
$obj = new Z();
echo $obj->foo();

What is the output?