That's what Citrix made NFuse for. NFuse allows users to access Citrix based application through the web browser. I don't think you can do it directly from PHP.
I believe that NFuse uses an ICA Plug-In to connect you to the Citrix Server. This plug-in is downloaded and installed the first time you want to connect to Citrix through the web.
Check out Citrix's web site for more info.