In PHP 5 anytime a variable containing an object is assigned to another variable it ends up being passed by reference.

<?php
class foo {
     var $bar = 10;

    function baz($val) {
        global $foo2;
        $foo2 = $this;
        $this->bar *= $val;
    }
}

$a = new foo();
$foo3 = $a;
$a->baz(2);
var_dump($a->bar == $foo2->bar); // PHP4: False PHP5: True
var_dump($a->bar == $foo3->bar); // PHP4: False PHP5: True
?>