"
UPDATE `users`
SET `postguest` = `postguest` + 1,
`lastpost` = ".time().",
`balans` = `balans` + 4
WHERE `id` = ".$user_id.";
"правилье кавычки будут вот такими
"
UPDATE `users`
SET `postguest` = `postguest` + 1,
`lastpost` = ".time().",
`balans` = `balans` + 4
WHERE `id` = ".$user_id.";
"