Is there a good (PHP) frontend for editing PO files
You don't need the translation application to be php just because the original app to be translated is written in php (although it could be easier to install and manage since you're familiar with it).
You could use Pootle (uses Python and Django framework). It does exactly what you need: a web interface for editing .po files. See for yourself an example for an Abiword translation. Just run your own copy as an intranet application.
Pootle is a user-friendly web portal that makes the translation process so much simpler. It allows online translation, work assignment, gives statistics and allows easy volunteer contribution.