Stress-testing web application

I have a worker.php that performs certain actions and uses mysql as a queue. Now I want to stress-test the setup and find out how many workers could run simultaneously before crashing/slowing down the server.

Especially the mysql part interests me.

How can I best setup a web interface where I can enter a number of workers hit go and it executes X workers?

I thought of just having curl_multi call the url of the worker with X "threads". This might be not ideal but imo the simplest solution.

Or would it best to have a C/C++ program that runs the command "php worker.exe" in X threads and call that program via a web interface?

Or is there something else?