Normally this code should print "0", 
however, when uncommenting a single empty method call, 
it outputs "1".
It seems like a copy suddenly becomes a reference.

<?php

$a1 = new A();
//$a1->c->f();  // uncomment this to expose error.
$a2 = $a1;
$a2->c->x = 1;
echo $a1->c->x;

class C {
    var $x = 0;
    function f() {}
}

class A {
    var $c;
    function A() { $this->c = new C(); }
}

?>

Thanks. 
Anahoret Team.