Fatal error: Can't use function return value in write context

Fatal error: Can't use function return value in write context in C:\xampp\htdocs\webframe\Classes\user.php on line 17

class User {
    public $username;
    public $ID;
    private $password;
    
    public function login($username,$password){
        $username = mysql_real_string_escape($username);
        $password = mysql_real_string_escape($password);
        $password = sha1(md5(sha1('randomshithere}{x!§')) + md5($password) + sha1('randomshithere'));
        $result = mysql_query('SELECT password,ID FROM users WHERE username = '.  $username.'AND password = '.$password);
        $row = mysql_fetch_assoc($result);
        if(mysql_num_rows($result) === 1){
            $this->password = $row['password'];
            $this->username = $row['username'];
            $this->ID = $row['ID'];
            $_SESSION('username') = $this->username;
            return 0;
        }
        else if(mysql_num_rows($result) === 0){
            return 1;
        }
        else{
            return 2;
        }
    }
    
    public function errorMessage($x){
        switch($x){
            case 0:
                return null;
                break;
            case 1:
                return 'Gebruikersnaam en / of paswoord is niet gevonden';
                break;
            case 2:
                return 'Er zijn meerde resultaten gevonden.';
                break;
            default:
                return 'De switch is kapot in de gebruikers klas bij funcitie errorMessage!';
        }
    }
}