OK, I did sort of figure out a possible solution using REST API:
ZCS 6.0:Zimbra REST API Reference - Zimbra :: Wiki
Right now, I don't need to post any data, only search. I figure with REST I could use php to make call to Zimbra server using either "curl", file_get_contents("http://www.example.com/file.xml") or http_get("http://www.example.com/file.xml").
However, I want to be able to search the Global Address List. My problem now is how to authenticate a user to search GAL using REST.
Any suggestions?