Is rewriting a PHP app into Python a productive step?
As others have said, re-writing will take a lot longer than you think and fixing all the bugs and making use everything worked like in the old version will take even longer.  Chances are you are better off simply improving and refactoring the php code you have. There are only a few good reasons to port a project from one language to another:

Performance.  Some languages are simply faster than others, and there comes a point where there is nothing left to optimize and throwing hardware at the problem ceases to be effective. 
Maintainability.  Sometimes it is hard to find good people who know some obscure language which your legacy code is written in.  In those cases it might be a good idea to re-write it in a more popular language to ease maintenance down the road.
Porting to a different platform.  If you all of a sudden need to make your old VB program run on OS X and Linux as well as Windows then you’re probably looking at a re-write in a different language

In your case it doesn't seem like any of the above points hold.  Of course if it's an unimportant app and you want to do it for the learning experience then by all means go for it, but from a business or economic point of view I'd take a long hard look at what such a re-write will cost and what exactly you hope to gain.
I have some old apps written in PHP that I'm thinking of converting to Python - both are websites that started as simple static html, then progressed to PHP and now include blogs with admin areas, rss etc. I'm thinking of rewriting them in Python to improve maintainability as well as to take advantage of my increase in experience to write things more robustly.
Is this worth the effort?