PHP multi thread experienced professional

I need a professional programmer who has experience in PHP multi thread in order to review an online application.
There are multi clients who log in and use the service and there are times that the server can not cope.
You will need to be able to test the processes and find which part of the code causes the server to overload.

Also users need to run a very intensive processing algorithm and you will need to make sure that this is run in a different thread so that it doesn't overload the current application.