Disclosure: I don't work with MS products and I haven't been keeping up with PHP as well as I used to. However, no one has responded, so FWIW:
I would be wary of libraries that purport to interface directly with MS products. Your best bet may be to export the Outlook calendar into a format readable by PHP, then sync from there. Dump the Outlook information every once in a while, and have PHP generate the interface from the dump.