Description:
------------
The built in function is_callable, does not return false, on unreachable functions like protected and private ones.

Reproduce code:
---------------
<?php
class test_class {
   private function test_func1(){}
   protected function test_func2(){}
}
$object = new test_class;

var_dump(is_callable(array($object,'test_func1')));
echo chr(10);
var_dump(is_callable(array($object,'test_func2')));
?>

Expected result:
----------------
bool(false)
bool(false)

Actual result:
--------------
bool(true)
bool(true)