Is rewriting a PHP app into Python a productive step?
As others have said, look at why you are doing it.
For instance, at work I am rewriting our existing inventory/sales system to a Python/django backend. Why? Because the existing PHP code base is stale, and is going to scale poorly as we  grow our business (plus it was built when our business model was different, then patched up to match our current needs which resulted in some spaghetti code)
So basically, if you think you're going to benefit from it in ways that aren't just "sweet this is in python now!" then go for it.
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?