$obj->test="test";
settype($obj->test2,"object");
$obj->test2->test="subobject test";
echo serialize($obj->test2);
echo "<br>";
echo serialize($obj);

prints out two different results in PHP 4.03pl1 and PHP 4.02. 
The settype command is the problem:
Without the settype command it works correct in PHP4.03pl1.