Languages have different pros and cons and also quite a bit of cons/pros depending on viewpoint.
PHP is quite good if you like messy code and typing all manner of brackets all over the place, it is still good if you don't. What I mean is, PHP works and you don't have to think too much about deployment issues, however, a lot of people burn themselves thinking that there are no deployment issues and on the lack of never looking at those issues.
Python is awesome in my opinion, it is clean and elegant. Of course as you say the end users, the students in africa as you say, won't know how elegant a language is behind their application. But they may notice if you make use of a few things that Python makes a lot easier compared to PHP, such as realtime cross client updates, this ofcourse requires javascript or Brython in the web browser but, hey, Python is great on the backend. With Brython you can use Python across your entire stack (backend, browser, phone apps)
Javascript is awesome too, if you, like php enjoy syntactic hell. This can be worked around using coffee script but that has it's own issues (debugging). JS can also run on the backend resulting in one language across your entire stack.
Those are just three languages and just a snippet of whats good about them. You can do realtime cross client updates with PHP too if you really feel like it, but it is not reccomended. You can run Javascript and Python Backends on anything you can run PHP on. Each of these languages are just as good really, it all boils down to one thing.
What can you use to produce solid, effective quality code in as litle time as possible?
Answer that and you have your answer about what is the best language.