I believe by design it will always be: $foo = $o->foo; $foo();

Using __get/__set behaves the same way. Let's say $o is of SomeType that defines those magic methods. You can have a function foo on that type that is defined as a public method.

    $o->foo(); // the actual instance method
    $o->foo = function() { print 'bar'; };
    $o->foo(); // same instance method
    $foo = $o->foo;
    $foo(); // prints bar

In the case of StdClass it might be possible to do it your way where foo could be callable ($o->foo()) but I can see why they wouldn't allow this in order to keep things consistent.

Edit: It also behaves the same way if you assign a function to a public field of a class (I'm using 5.4) so I actually support not allowing a method to be directly callable in this fashion because this would be the only case where this behavior would occur.