Mit onlinebrugersystem vil ikke virke

PHP kode 

 // ONLINE USERS
 
 $db_checkifusersonline = mysql_query("select * from users");
 $checkifusersonline = mysql_fetch_array($db_checkifusersonline);
 
 // CHECK OTHER USERS TIME
 
 $timeisout = $now+"100";
 mysql_query("update users set online = '0' where lastactive =< '$timeisout'");
 
 // SET ONLINE USERS TIME
 $now = date("H").date("i").date("s");
 mysql_query("update users set lastactive = '$now', online = '1' where id = '$useronline[id]'");