PHP is almost completely used as a server side language for websites, both big and small.
Python is a general purpose programming language used to make GUI's or many other things that a general programming language could be used for.
PHP was originally designed to be embedded in HTML. It was implemented as a module that gets loaded into a web server. It therefore doesn't need tons upon tons of stuff. For just a few examples: it gets daemonization from the web server, so it doesn't need that. It gets the GUI skeleton from the browser, so it doesn't need that. It didn't even initially have support for running outside of a browser at all - I *believe* the ability to write a command line script with it came late in the 3.x or very early 4.x versions. It didn't have socket support until (iirc) 4.x, because any socket-level stuff was initially assumed to be HTTP operations handled by a web server.
Python, on the other hand, was built by a systems programmer to be object oriented from the ground up and include a standard library of modules and classes that exposed behaviors, with relatively few standalone functions. It was not built with any really hard assumptions about the use cases it would support. So whereas PHP was initially designed as a 'web development' language, Python was designed as a 'general purpose' language. To this day, while there are all kinds of experiments around replacing javascript in the browser with Python, and using Python embedded in HTML a la PHP, these remain in the "novelty" category as far as I know.