CREATE TABLE `tbl` (
  `id` int unsigned NOT NULL AUTO_INCREMENT,
  `data` varchar(100) NOT NULL,
  `hash` varchar(10) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `knock` (`knock`) USING BTREE
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

$hash = sanitize($_GET['hash']);
$res = $mysqli->query("SELECT `id`, `data` FROM `$sql_table` WHERE `knock`< NOW() - INTERVAL 300 SECOND LIMIT 100");
$ids = array();
if ($res) {
    while ($row = $res->fetch_row()) {
        echo implode(" ", $row)."\n";
        $ids[] = $row[0];
    }
    $res->free();
}
if (isset($ids) && count($ids) != 0) {
    $mysqli->query("UPDATE `$sql_table` SET `knock`=NOW(), `hash`='$hash' WHERE `id` IN ('".implode("','", $ids)."')");
}