Código:
Gracias por su ayuda. $antiflood_time = time() + 20;
$cookie = explode(",", $_COOKIE['antiflood']);
if ($cookie[1] >= $cmensaje)
{
echo "<script>alert(\"$error_antiflood\")</script><body onload=\"javascript:window.history. go( -1 )\">";
exit;
}
elseif ($cookie[0] == $_SERVER['PHP_SELF'])
{
$cookie[1]++;
setcookie("antiflood", $_SERVER['PHP_SELF'] . "," . $cookie[1], $antiflood_time);
}
else
{
setcookie("antiflood", "");
setcookie("antiflood", $_SERVER['PHP_SELF'] . ",1", $antiflood_time);
}

