You support calling user methods using an array in the call_user_func() function. But related functions of call_user_func() don't support the syntax. I would like to see that function_exists could check if the user method exists. 

The following example code should work:

class Foo {
  function bar($what = 'world') {
    print 'Hello ' . $what;
  }
}

$a_foo = new Foo();

$user_method = array($a_foo, 'bar');

if (!function_exists($user_method)) 
  die('You shouldn\'t see this :)');

$user_method('PHP'); // Should print "Hello PHP" ofcourse.

Furthermore the implementation of the user_method calling in call_user_func is incorrect for referencing variabeles, which might be pretty usefull. (see http://bugs.php.net/bug.php?id=17309)