Description:
------------
Analog to __toString() a __toInt() method.

Additionally allow NULL as a valid return value for casting methods.

Test script:
---------------
class test {
   private $value;
   function __construct($value = null) {
        $this->value = $value;
   }
   function __toString() {
      return $this->value ? (string) $this->value : $this->value;
   }
   function __toInt() {
     return $this->value ? (int) $this->value : $this->value;
   }
}

(string)(new test()) === null;
(string)(new test('1')) === '1';
(string)(new test(1)) === '1';
(int)(new test()) === null;
(int)(new test('1') === 1;
(int)(new test(1) === 1;


Expected result:
----------------
6 x true

Actual result:
--------------
* Catchable fatal error: Method test::__toString() must return a string value in php shell code on line 1

*__toInt is never called