<?php

 $data = 'Ваш путь к файлу';
 $time = time ();
 $past_time = time ()-600;

 $readdata = fopen ($data, 'r') or die("Не могу открыть файл $data");
 fclose ($readdata);
 $data_array = file ($data);

 if (
      getenv ('HTTP_X_FORWARDED_FOR')
    ) {
        $user = getenv ('HTTP_X_FORWARDED_FOR');
      } else {
               $user = getenv ('REMOTE_ADDR');
             };

 $d = count ($data_array);
 for ($i=0; $i<$d; $i++) {
                           list ($live_user, $last_time) = explode ('::', $data_array[$i]);
                           if (
                                $live_user != '' &&
                                $last_time != ''
                              ) {
                                  if (
                                       $last_time < $past_time
                                     ) {
                                         $live_user = '';
                                         $last_time = '';
                                        };
                                  if (
                                       $live_user != '' &&
                                       $last_time != ''
                                     ) {
                                         if (
                                              $user == $live_user
                                            ) {
                                                $online_array[] = "$user::$time\r\n";
                                              } else {
                                                       $online_array[] = "$live_user::$last_time";
                                                     };
                                       };
                                };
                         };

 if (
      isset ($online_array)
    ) {
        foreach (
                  $online_array as $i => $str
                ) {
                    if (
                         $str == "$user::$time\r\n"
                       ) {
                           $ok = $i;
                           break ;
                         };
                  };
        foreach (
                  $online_array as $j=>$str
                ) {
                    if (
                         $ok == $j
                       ) {
                           $online_array[$ok] = "$user::$time\r\n";
                           break ;
                         }
                  };
      };

 $writedata = fopen ($data, 'w') or die ("Не могу открыть файл $data");
 flock ($writedata, 2);
 if (
      $online_array == ''
    ) {
        $online_array[] = "$user::$time\r\n";
      };
 foreach (
           $online_array as $str
         ) {
             fputs ($writedata, $str);
           };
 flock ($writedata, 3);
 fclose ($writedata);

 $readdata = fopen ($data, 'r') or die ("Не могу открыть файл $data");
 $data_array = file ($data);
 fclose ($readdata);
 $online = count ($data_array);

 echo "На сайте: $online";

?>