(try to) optimize a little, no need for mysql to count that, we can do it just fine

forum/Sources/Subs.php

@@ -1432,9 +1432,10 @@ function writeLog($force = false)
 	// Guests use 0, members use their session ID.
 	$session_id = $user_info['is_guest'] ? 'ip' . $user_info['ip'] : session_id();
 
+    $sess_expire_time = time() -  ($modSettings['lastActive'] * 60);
 	db_query("
 		DELETE FROM {$db_prefix}log_online
-		WHERE logTime < NOW() - INTERVAL " . ($modSettings['lastActive'] * 60) . " SECOND
+		WHERE logTime < '{$sess_expire_time}'
 			OR session = '$session_id'" . (empty($ID_MEMBER) ? '' : " OR ID_MEMBER = $ID_MEMBER"), __FILE__, __LINE__);
 	db_query("
 		INSERT IGNORE INTO {$db_prefix}log_online