Description:
------------
When extending the class tidy (http://docs.php.net/class.tidy) with a subtype MyTidy, the PHP engine fails type checks for MyTidy (probably easier to explain using code).

This is inconsistent with normal behavior in the object model. is_a($o, get_class($o)) should always be true if $o is an object, that is an object should always be an instance of its own class.

Reproduce code:
---------------
<?php
class MyTidy extends tidy
{
    // foo
}

function doSomething(MyTidy $o) 
{
    var_dump($o);
}

$o = new MyTidy();

var_dump(is_a($o, get_class($o)), get_class($o));
doSomething($o);

Expected result:
----------------
bool(true)
string(7) "MyTidy"
object(MyTidy)#1 (2) {
  ["errorBuffer"]=>
  NULL
  ["value"]=>
  NULL
}

Actual result:
--------------
bool(false)
string(6) "MyTidy"
PHP Catchable fatal error:  Argument 1 passed to doSomething() must be an instance of MyTidy, instance of tidy given, called in /home/daniel/test.php on line 16 and defined in /home/daniel/test.php on line 7

Catchable fatal error: Argument 1 passed to doSomething() must be an instance of MyTidy, instance of tidy given, called in /home/daniel/test.php on line 16 and defined in /home/daniel/test.php on line 7