Is rewriting a PHP app into Python a productive step?
You need to take some parts into mind here,

What will you gain from re-writing
Is it an economically wise decision
Will the code be easier to handle for new programmers
Performance-wise, will this be a good option?

These four points is something that is important, will the work be more efficient after you re-write the code? Probably. But will it be worth the cost of re-development?
One important step to follow, if you decide to re-write, make 3 documents, first Analyze the project, what needs to be done? How should everything work? Then put up a document with Requirements, what specificly do we need and how should this be done? Last but not least, the design document, where you put all your final class diagrams, the system operations and how the design and flow of the page should work.
This will help a new developer, and old ones, to actually think about "do we really need to re-write?".
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?