<?php if ($db->query("SELECT `id` FROM `mail` WHERE `id_user` = '" . $user['id'] . "' AND `read` = '0'")->num_rows > 0) { die("+"); }
<?php if (mysql_num_rows(mysql_query("SELECT `id` FROM `mail` WHERE `id_user` = '" . $user['id' ] . "' AND `read` = '0'")) > 0) { die("+"); }