How to create buzzer functionality in php-javascript quiz game?

I’m making little quiz game and want to add online multiplayer functionality. I want to add a buzzer – who presses key first, answers question first. So far everything is in javascript and php and works fine. I guess app firstly needs some kind of logins and then while players are logged in it needs to react on events from many ips simultaneously. I suppose there is no short answer to this problem, so I’ll appreciate any guidelines on where to start, because this is new territory for me.

Thanks in advance.

Source: stackoverflow-php