Python will allow for easier modularity, wheras PHP will make it easier to pollute the global namespace. Likewise, all standard python components are made available in namespaced modules, are consistently named and well documented. You can certainly write PHP without creating a global mess, but novice developers will be tempted to introduce globals and the standard library is in the global namespace, setting a bad example.

Perhaps more importantly, many skilled developers favor Python and will avoid PHP. When you seek to grow your team, you will likely draw from a better talent pool if your application is written in Python.