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xoops will not allow you to use $xoopsDB->query() with $_GET for security reasons as John mentioned.. you have to use queryF() with $_GET which is insecure.
queryF basically forces the query which bypasses all sanitization in the query leaving the query itself open to exploitation. you should only ever use queryF if you have no other choice at all, and then i wouldn't recommend using it on any pages/forms that are accessible by users.
if you can change from $_GET to use $_POST instead, then $xoopsDB->query() will function correctly.
as david mentioned, can you not use >
$cid = isset($_REQUEST['cid']) ? $_REQUEST['cid'] : 0;
& use a class/function to execute the query itself instead of having the query execute in the same script that the form uses? then return the results. i'm no expert tho..