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//alte user löschen
DELETE FROM online WHERE lasttime < (UNIX_TIMESTAMP() - 5*60)

//aktiven user suchen
SELECT uid FROM online WHERE ip = '$REMOTE_ADDR'
if(mysql_num_rows() > 0) {
   UPDATE online SET lasttime = UNIX_TIMESTAMP() WHERE uid = $row[0]
} else {
   INSERT INTO online(ip, lasttime) VALUES('$REMOTE_ADDR', UNIX_TIMESTAMP())
}

//anzahl der user online
SELECT count(uid) FROM online
echo "Hurra! $x user online!"
}