PHP for websites. Python for pretty much anything else, such as commandline tools, long-running scripts, daemons, etcetera. If you re writing a PHP script and you re reaching for functions in the posix extenstion, shared memory or other low-level stuff then that s generally a sign that Python would be better suited. It s not that PHP can t do it, but Python just does it better and less buggy.
Especially when you re venturing into background daemons for your website you ll want to look at Python. PHP has some garbage collection problems in long running processes such as daemons. Also, some functionality is much easier and clearer in Python (e.g. redirecting STDIN, STDOUT and STDERR. PHP misses posix_dup2()). Also, Python has threads :-)
The only time when I now use PHP background daemons for my websites is when they can re-use significant amounts of code (such as with MVC frameworks like CakePHP).
One more advantage of Python is that there are many, many libraries for it, because it s rather easy to create a Python wrapper for a C library. So, Python has libraries that PHP doesn t have (OpenGL, multimedia, etcetera). So if you re into those areas Python becomes the obvious choice.