Contacts / address book data feed import for MS Exchange

Question

Hi

I've got an intranet (PHP / Zend Framework) application that holds people's contact information (name, address, phone, email, skype etc)

I'm looking for a way to (securely!) create a data feed that MS Exchange (preferably) or MS Outlook (second choice) can consume that will ensure our worldwide user base always has up to date contact details right in their email client.

This is not a bulk import: its a data feed that must keep contact details for 800 people globally in synch across 20 offices worldwide (many with Exchange servers, some using other stuff which I'll deal with later). Would prefer not to have to do any programming on the Exchange side if possible.

thanks!