<?php

    class mySQL
    {

        /*
         * You know this shit by now, Hopefully.
         */

    }

    class user
    {

        private $userId = 0, $loggedIn = false;

        public function __construct()
        {

            if( $_POST )
            {

                $this->_logIn();

            }

            $this->_checkSession();

        }

        public function isLoggedIn()
        {

            return $this->loggedIn;

        }

        private function _logIn()
        {

            $query = mysql_query('SELECT id FROM users WHERE username = "'.$_POST['username'].'" AND password = "'.$_POST['password'].'" LIMIT 1;');

            if( mysql_num_rows( $query ) > 0 )
            {

                $this->userId = mysql_result( $query, 0, 0 );

                mysql_query('INSERT INTO sessions (sessionId, userId)VALUES("'.session_id().'", "'.$this->userId.'");');

            }
            else
            {

                /*
                 * you will not to do some more validation
                 *
                 */

            }

        }

        private function _checkSession()
        {

            $query = mysql_query('SELECT userId FROM sessions WHERE sessionId = "'.session_id().'" LIMIT 1;');

            if( mysql_num_rows( $query ) > 0 )
            {

                $this->loggedIn = true;

            }

        }

    }