Try:
$clientipaddress = $_SERVER['REMOTE_ADDR'];
$nowday = date("Y-m-d", mktime());
$thissessionid = session_id();
@mysql_query("UPDATE '$prefix'online SET session_id = '$thissessionid', lastactive='$nowday', member='$member', memberid='$memberid' WHERE ip_address = '$clientipaddress'");
mySQL query strings don't like it much when you add array values, or global variables like you did. Assign them to variables first, and use those variables in your SQL.