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.
Python's a very good language to learn for a number of reasons, including its clear syntax, its excellent standard library, its multi-paradigm support, and the helpful community that surrounds it. 
However, scalability has more to do with your system's overall architecture than with the programming language you choose. Often, the database is the main bottleneck, and the performance of the programming language that's talking to the database will not have much impact on overall site performance.
Would porting parts of a PHP application to Python help it scale?