Serverless is too overloaded a term to have any meaning. I'm not really seeing how Python or PHP "scales infinitely" in any way that C#, Java, C++ couldn't.
For large PHP setups it is usually the number of database connections that is the limiting factor, however that is why historically the replicated MySQL databases was such a good fit for PHP, thus only creating a limit for writes on the master.
For pretty much every modern programming language, IO is the bottleneck over everything else.
To save you some time, there are practically no metrics in which I think PHP beats another programming language other than maturity, and even then, not really.