Load test server with PHP script?

I think this rather vague question can be attributed to programming. I did consider if it belonged in the Ask Ubuntu / WordPress Development / Linux Administration communities, but of course I don't like spamming either. Apologize if I made a huge mistake and caused you a bad day.

Take a Linux server, there are several WordPress sites on it and we can already guess that the dominant interpreted language is PHP when you consider the scenario above.

These are not live sites, but simply sites copied from the production server.

I want to simulate random traffic for all of them at once, to check if the server can handle the load. Perhaps a PHP script can do this from the command line?

Just look for ideas at this stage.

I tried https://www.blitz.io/ - works great, however only one site at a time as far as I know.