what server-side language to use for multiplayer card game
[Full disclosure: I am the co-founder of Lightstreamer]
With Lightstreamer, you can stick to PHP for your server-side development. To develop your Lightstreamer Adapters (the custom server-side code needed by Lightstreamer) APIs are provided for: Java, .NET, Node.js, and plain TCP sockets. The TCP socket option means that your Adapter can talk to the Lightstreamer Server by implementing a simple protocol based on TCP (fully documented). This way, your can develop your Lightstreamer Adapters in PHP, which talk to the Lightstreamer Server via TCP. The Lightstreamer Server will take care of delivering the real-time messages to the actual clients, with scalable fan-out, low latency, and passing though any possible type of proxy and firewall.
Both Lightstreamer or Node.js are more than capable of what you are looking for.
I use lightstreamer day in and day out for spread betting, CFDs and stock trading. [Full disclosure. The company I work for is probably alessandroalinones biggest client ;)] These traders require millisecond latency and high security and lightstreamer does the job.
We are researching now between using Lightstreamer with PHP adapter and between using Ratchet PHP. Ratchet seems to be more documented and easier to use, while Lightstreamer seems to be capable of more things, however it doesn't have any official PHP adapter built yet.
The popularity, the documentation and the live examples are some of the most important things that we are considering in making our decision.