Description:
------------
When using type-hinting in a function's arguments, PHP should allow a default value of NULL. Otherwise you have to manaully check the type with instanceof. This would make it more convenient, and more consistent with Java.

Reproduce code:
---------------
function f (MyClass $o = NULL) {
  echo "success";
}

f();

Expected result:
----------------
success

Actual result:
--------------
PHP compiler error.