Try:
<?php
if ((((isset($_POST["doc1del"]))?$_POST["doc1del"]:"") != "")) {
$UpdateQuery = new WA_MySQLi_Query($conn_mdb1);
$UpdateQuery->Action = "update";
$UpdateQuery->Table = "maintbl";
$UpdateQuery->bindColumn("doc1AltText", "s", "", "WA_BLANK");
$UpdateQuery->addFilter("mainID", "=", "i", "".($_GET['mainID']) ."");
$UpdateQuery->execute();
$UpdateGoTo = "";
if (function_exists("rel2abs")) $UpdateGoTo = $UpdateGoTo?rel2abs($UpdateGoTo,dirname(__FILE__)):"";
$UpdateQuery->redirect($UpdateGoTo);
}
?>