I appreciate there are perhaps better ways of making a PHP application scale than the above, however I'm wondering more on principle. From what I've heard, Python is faster than PHP and I'm trying to decide which language to learn next, as a PHP developer.
I don't think rewriting in Python will help you that much, it is very hard to say anyway without knowing your exact scalability problem. When it comes to real scaling issues, every problem is unique and there is no one true solution.
That being said, Python is an interesting language, and if you've time on your hands and want to learn something, it sounds like a good idea to look at it closely. I would also look at JavaScript more closely though, be it using NodeJS (which is also pretty damn fast for some things) or just in a browser, it's a language that is not going away anytime soon. If you're a web developer, you can't know enough JS imo.
Would porting parts of a PHP application to Python help it scale?