Generally, Java servlets live in a servlet container, where PHP pages
are executed through CGI.  It completely depends on the platform you're
using on how to combine the two.  I believe resin can do both without
too much work, as it supports J2EE and PHP

Easy. Do a client-side redirect from one to the other. You can pass any
arguments you like in the redirect URL, although this isn't very secure as
they will go over the network in clear twice. Or you can create a uniquely
named temporary file on the server, and pass it's name as an argument in
the redirect URL.

       HttpServletResponse res =
               (HttpServletResponse) context.get( RESPONSEMAGICTOKEN );
       HttpServletRequest req =
               (HttpServletRequest) context.get( REQUESTMAGICTOKEN );
       URL desturl = null;

       if ( req != null ) // it *shouldn't* be..
       {
               // normalise the URL if possible (should be)
               URL here = new URL( req.getRequestURL(  ).toString(  ) );

               desturl = new URL( here, deststring );
       }
       else
       {
               desturl = new URL( deststring );
       }

       // convert it back into a string
       deststring = desturl.toString(  );

       res.setStatus( HttpServletResponse.SC_MOVED_TEMPORARILY );
       res.setHeader( "Location", deststring );

You can do the equivalent in PHP but seeing it's ten years since I built
anything significant in PHP I'll leave that to others.