$query = mysql_query("SELECT * FROM online WHERE sessionid='".mysql_escape_string( session_id()) ."' AND sessionip='{$_SERVER['REMOTE_ADDR']}'");
$result = mysql_fetch_array( $query );
if ( ($result['sessionid'] !== session_id() ) && ( $result['sessionip'] !== $_SERVER['REMOTE_ADDR'] ) ){
header('Location:Login.php');
}

something like that to check if some one is logged in or not?