Objects must be explicitly defined as references in PHPv4, otherwise they will be copy-on-write like all other PHP variables.

<?php
class simple {
    var $name;
}

function changeName (&$o) {
    $o->name = "Sterling Hughes";
}

$o = &new simple;
$o->name = "Johnny Red";
echo $o->name . "\n<br />\n";
changeName($o);
echo $o->name . "\n<br />\n";
?>
Output