Consider the following script:

<?
class test 
{
	var $x;
	var $y;
	
	function test ()
	{
		$this->x = 0;
		$this->y = 0;
	}
	
	function set ($x, $y)
	{
		$this->x = $x;
		$this->y = $y;
	}
	
}
$mumu = array();
$gaga = array();

for($i=0;$i<3;$i++)
{
	$test = new test ();
	$test->set($i, $i);
	$mumu["xx".$i] = $test;
}

for($i=0;$i<5;$i++)
{
	$test = new test ();
	$test->set($i, $i);
	$gaga["xx".$i] = $test;
}

$fff = array_diff($mumu, $gaga);
$ggg = array_diff($gaga, $mumu);

echo "<pre>";
print_r($fff);
echo "</pre>";

echo "<pre>";
print_r($ggg);
echo "</pre>";
?>



the following is the output from a server with php 
4.0.4pl1:
Array
(
)
Array
(
    [xx3] => test Object
        (
            [x] => 3
            [y] => 3
        )

    [xx4] => test Object
        (
            [x] => 4
            [y] => 4
        )

)


and the following is the ouput with php 4.0.5:
Array
(
)
Array
(
)

the php.ini is the same on both servers.