Redis server to use for testing.

Some example values:

* testredis.abc.com - To connect to a Redis server by hostname (Port 6379 by default).
* testredis.abc.com:1234 - To connect to a Redis server by hostname with a specific port.
* 1.2.3.4 - To connect to a Redis server by IP address (Port 6379 by default).
* 1.2.3.4:1234 - To connect to a Redis server by IP address with a specific port.
* unix:///var/redis.sock - To connect to a Redis server using a Unix socket.
* /var/redis.sock - To connect to a Redis server using a Unix socket (alternative format).

If cluster mode is enabled, specify servers separated by a new line, for example:<br>
  172.23.0.11<br>
  172.23.0.12<br>
  172.23.0.13<br>

For further information, see <a href="https://redis.io/docs/reference/clients/#accepting-client-connections">Accepting Client Connections</a> and <a href="https://redis.io/resources/clients/#php">Redis PHP clients</a>.