If so, there are a number of approaches to take. If your app is blocking on sending/receiving data to/from the client, it would benefit you to segregate the actual app servers from the frontend servers. You can use mod_fastcgi and run your PHP stuff in FastCGI, or you can use a frontend threaded apache talking to a backend PHP apache via mod_proxy.
If you are blocking on things like database access or other non-web IO, then you need an event loop so your app can do something else while waiting for IO. I don't know what your options are in the PHP world, but this is very easy to do in Perl.
Finally, real threads will probably also solve your problem. I recommend CL or Haskell. (Although I think Slicehost still uses an ancient Xen that can't handle SBCL. Oops.)