之前配置过redis，本地线上都没问题，但是这次本地没问题，线上却死活连不上
报错信息甚至都是"Failed to open redis DB connection (xxx): - "
坑爹啊，居然没有显示错误编码和错误错误信息！
查看yii-redis Connection源码，找到redis连接方法：

public function open()
    {
        if ($this->_socket !== false) {
            return;
        }
        $connection = ($this->unixSocket ?: $this->hostname . ':' . $this->port) . ', database=' . $this->database;
        \Yii::trace('Opening redis DB connection: ' . $connection, __METHOD__);
        $this->_socket = @stream_socket_client(
            $this->unixSocket ? 'unix://' . $this->unixSocket : 'tcp://' . $this->hostname . ':' . $this->port,
            $errorNumber,
            $errorDescription,
            $this->connectionTimeout ? $this->connectionTimeout : ini_get('default_socket_timeout'),
            $this->socketClientFlags
        );
        if ($this->_socket) {
            if ($this->dataTimeout !== null) {
                stream_set_timeout($this->_socket, $timeout = (int) $this->dataTimeout, (int) (($this->dataTimeout - $timeout) * 1000000));
            }
            if ($this->password !== null) {
                $this->executeCommand('AUTH', [$this->password]);
            }
            if ($this->database !== null) {
                $this->executeCommand('SELECT', [$this->database]);
            }
            $this->initConnection();
        } else {
            \Yii::error("Failed to open redis DB connection ($connection): $errorNumber - $errorDescription", __CLASS__);
            $message = YII_DEBUG ? "Failed to open redis DB connection ($connection): $errorNumber - $errorDescription" : 'Failed to open DB connection.';
            throw new Exception($message, $errorDescription, $errorNumber);
        }
    }

调试了半天，也没能打印出errorDescription;
最后突然注意到$this->_socket = @stream_socket_client()
晕。。。@符把stream_socket_client()函数错误信息给屏蔽了
去掉@符号，显示是因为php.ini把stream_socket_client()函数给禁用了，修改php.ini重启，redis可以用了==！
一个弱智的问题浪费了2个小时，以后遇到问题一定要多看底层源码，注意细节