常量未定义错误

Question

我试图在其中一个php登录库中定义自己的函数。 定义了一些常量，它们完全正常，直到我自己调用databaseConnection（）函数。

 private function databaseConnection()
{
    // if connection already exists
    if ($this->db_connection != null) {
        return true;
    } else {
        try {
            // Generate a database connection, using the PDO connector
            // @see http://net.tutsplus.com/tutorials/php/why-you-should-be-using-phps-pdo-for-database-access/
            // Also important: We include the charset, as leaving it out seems to be a security issue:
            // @see http://wiki.hashphp.org/PDO_Tutorial_for_MySQL_Developers#Connecting_to_MySQL says:
            // "Adding the charset to the DSN is very important for security reasons,
            // most examples you'll see around leave it out. MAKE SURE TO INCLUDE THE CHARSET!"
            $this->db_connection = new PDO('mysql:host='. DB_HOST .';dbname='. DB_NAME . ';charset=utf8', DB_USER, DB_PASS);
            return true;
        } catch (PDOException $e) {
            $this->errors[] = MESSAGE_DATABASE_ERROR . $e->getMessage();
        }
    }
    // default return
    return false;
}

这是我定义的功能..

 private function updateLastLoginDate($user_name)
{
    if($this->databaseConnection())
    {
    $sth = $this->db_connection->prepare('UPDATE users '
                    . 'SET last_login_date = UTC_TIMESTAMP()'
                    . 'WHERE user_email =:user_name OR user_name =:user_name');
            $sth->execute(array(':user_name' => $user_name));
    }
}

当我调用这个函数时，错误表明databaseConntion函数中的所有常量都没有被定义..但除了我对我定义的函数的调用之外，它们工作得非常好...... 虽然我不喜欢pdo。