ช่วยแก้ไข code หน่อยคับมันขึ้น Using $this when not in object context

       if(!$this->isDataValid())
        throw new exception ('data');

<?php
class Login
{
    private $_id;
    private $_username;
    private $_password;
    private $_passmd5;
    
    private $_errors;
    private $_access;
    private $_login;
    private $_token;
    
    public function __construct()
    {
        $this->_errors = array();
        $this->_login = isset($_POST['login']) ? 1 : 0;
        $this->_access = 0;
        $this->_token = $_POST['token'];
        
        $this->_id = 0;
        $this->_username = ($this->_login) ? $this->filter($_POST['username']) : $_SESSION['username'];
        $this->_password = ($this->_login) ? $this->filter($_POST['password']) : '';
        $this->_passmd5 = ($this->_login) ? md5($this->_password) : $_SESSION['password'];
    }
    
    public function isLoggedIn()
    {
      ($this->_login) ? $this->verifypost() : $this->verifySession();
      return $this->_access; 
    }
    public function filter($var)
    {
     return preg_replace('/[^a-zA-Z0-9]/','',$var);  
    }
    public function verifypost()
    {
     try
     {
        if(!$this->isTokenValid())
            throw new exception ('Submit');
            
        if(!$this->isDataValid())
        throw new exception ('data');
        
        if(!$this->verifyDatabase())
        throw new exception ('username / password');
        
        $this->_access = 1;
        $this->signupSession();
     }
     catch(exception $e)
     {
      $this->_errors[] = $e->getMessage();  
     }   
    }
    public function verifySession()
    {
     if($this->sesstionExist() && $this->verifyDatabase())
     $this->_access = 1;   
    }
    public function verifyDatabase()
    {
        mysql_connect("localhost", "rosot", "1234") or die (mysql_error());
        mysql_select_db("payment") or die (mysql_error());
        
        $data = mysql_query("SELECT *  FROM `account` WHERE username = '($this->_username)' AND password = '($this->_passmd5)'");
        mysql_query($data) or die (mysql_error());
    }
    public function isDataValid()
    {
        return(preg_match('/^[a-zA-Z0-9]{5,12}%/',$this->_username) && preg_match('/^[a-zA-Z0-9]{5,12}%/',$this->_password))? 1 : 0 ;
    }
    public function isTokenValid()
    {
        return(!isset($_SESSION['token']) || $this->_token != $_SESSION['token']) ? 0 : 1;
    }
    public function signupSession()
    {
        $_SESSION['user_id'] = $this->_id;
        $_SESSION['username'] = $this->_username;
        $_SESSION['password'] = $this->_passmd5;
    }
    public function sesstionExist()
    {
        return (isset($_SESSION['username']) && isset($_SESSION['password']))? 1 : 0 ;
    }
    public function showErrors()
    {
    echo "<h3>Errors</h3>";
    
    foreach($this->_errors as $key=>$value)
    echo $value."<br>";    
    }
}
?>