None of our PHP scripts are working anymore in this PHP version!
$o->name="test";

$x=serialize($o);
$o=unserialize($x);
echo $o->name;

Prints out a "Fatal error: The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition stdClass of the object you are trying to operate on was
loaded _before_ the session was started in /usr/local/apache/htdocs/test.php3 on line 9"

The following exaples are working:

 $o->name="test";
echo $o->name;
and 
class test {
}
$o=new test;
$x=serialize($o);
$o=unserialize($x);
echo $o->name;
In PHP 4.0.0 this works ok!