The following code was responsible for this unusual behavior.

if ($_POST['limit'])
{
$limit= $_POST['limit'];
}
elseif ($_GET['limit'])
{
$group_id = $_GET['limit'];
}

Unable to understand why it created such a problem (I noticed adding amp; to the & was helpful, but there should not be any need to add such amp; to and & generated by a form, and not by the code us), I simply commented it out.

Lorenzo, as it was yours, maybe you know better was could be the cause of trouble.
I suspect this page will no longer works with register globals off.

Almost off-topic:
In fact, I suspect getting register globals off to be achievable only with a recently started PHP application. And, well, the major advantage of PHP was simplicity. But with registers global set to off, it definitely no longer have any advantage on perl.
Reading http://tnx.nl/php just confort my opinion.