You are going to want to define what you want each player to do. Flash Builder is probably going to be on the client side. PHP, although it can do client side work, is going to be on the server side, assuming "normal" implementation.
So what do you want PHP to do. Again, I am going to assume the "normal" and you want PHP to serve up data. One of the easiest ways to integrate that data into Flash Builder is via JSON so this article handles the PHP side
http://www.sencha.com/learn/Tutorial:Creating_JSON_Data_in_PHP
and this article handles the Flash Builder side
http://sujitreddyg.wordpress.com/2009/06/05/consuming-json-using-data-centric-development-feature-dcd-in-flash-builder-4/
There are other ways to pass data (XML, say), but until you decide what it is that you are trying to do, exactly, the above should get you started.