làm thế nào đếm số người đang online trongPHP

<?php
class UsersOnline {
    /* Các tham số kết nối được sử dụng để dùng chung */
    var $host     = 'localhost';
    var $database = 'gido';
    var $user     = 'root';
    var $password = 'password';
    var $timeoutSeconds = 120;
    var $numberOfUsers = 0;
    function UsersOnline() {
        $this->refresh();                                                                              
    }
    function getNumber() {
        return $this->numberOfUsers;
    }
    function printNumber() {
        if($this->numberOfUsers == 1) {
            echo "$this->numberOfUsers";
        } else {
            echo "$this->numberOfUsers";
        }
    }
    function refresh() {
        global $REMOTE_ADDR, $PHP_SELF;
        $currentTime = time();
        $timeout = $currentTime - $this->timeoutSeconds;
        mysql_connect($this->host, $this->user, $this->password)
            or die('Error conecting to database');
        mysql_db_query($this->database,
                       "INSERT INTO usersonline VALUES ('$currentTime','$REMOTE_ADDR','$PHP_SELF')")
            or die('Error writing to database');                      
        mysql_db_query($this->database,
                       "DELETE FROM usersonline WHERE timestamp < $timeout")
            or die('Error deleting from database');          
        $result = mysql_db_query($this->database,
                                 "SELECT DISTINCT ip FROM usersonline WHERE file='$PHP_SELF'")
            or die('Error reading from database');
        $this->numberOfUsers = mysql_num_rows($result);                                                                                   
        mysql_close();
    }
}
?>