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GET as opposed to POST is a method for getting variables from the user - such as when submitting a form, the information in the form is sent via either POST or GET.
GET parameters can also be sent via the URL (such as this viewtopic.php?topic_id=28295 - in the PHP file, this can be retrieved as $_GET['topic_id'] with the value 28295)
XOOPS Has an built-in security to disallow changes to the database, if the method used for transmitting is not POST - as POST parameters cannot be given in the URL. Therefore, either change your script to use the POST method in the form on your admin page - or (only if you are absolutely sure that you are not opening up for just anybody to do anything) use $xoopsDB->queryF() instead of $xoopsDB-query()