Amazanon MySQL RDS - 无法连接（SQLSTATE [HY000] [2003]）

Question

我正在尝试从Amazon云实例连接到RDS，并且我一直收到此错误

SQLSTATE[HY000] [2003] Can't connect to MySQL server on 'xxxxx' (4)

我可以通过MySQL Workbench连接到RDS而没有任何问题，但是当我尝试从实例连接时，会发生错误。这是连接到RDS的代码：

$this->_host     = 'xxx';
        $this->_user     = 'xxx';
        $this->_password = 'xxx';
        $this->_dbname   = 'xxx';

        $dns = 'mysql:host=' . $this->_host . ';dbname=' . $this->_dbname . ';port=3306';

        $options = array(
            \PDO::ATTR_PERSISTENT => true,
            \PDO::ATTR_ERRMODE    => \PDO::ERRMODE_EXCEPTION
        );

        try {
            $this->handler = new \PDO($dns, $this->_user, $this->_password, $options);
        } catch (\PDOException $e) {
           die($e->getMessage());
        }