I want to make an application where users use their own computer together with a host, the host click ”Start” and then there shall automatically show a button on all users computers at the same time. Then the first person who click the button will win. I want this to happen in the browser, but I dont know wich technology to use. I already know PHP and mysql, but I dont know anyway to update users computers in realtime. Wich technology would be the best choice to make this happen?
The solution here is basically web sockets, likely with a pub/sub layer on top. It can be done relatively simply with a decent Javascript and server-side library. PHP isn't the ideal language for this, but it works just fine with the right tools. Ratchet is a decent PHP web socket server implementation, and Autobahn|JS a decent client-side library (note: at the time of writing the latest Autobahn|JS WAMP implementation is incompatible with the older WAMP implementation of Ratchet, use Autobahn|JS WAMP v1). Follow the Ratchet tutorial, then expand into setting up a pub/sub server as described here (you don't need the ZeroMQ components, you'll be triggering events by a publish action instead of an external ZeroMQ event).
That's a 30,000 foot overview, go forth and try it.
Pusher.com has the ideal solution for this. You can send events, listen to these events and then respond accordingly. They have a free plan which I think is way more than you will probably need. Pusher works with JavaScript and it's extremely simple to get started
I suggest reading up on the documentation at pusher.com/docs