Try doing some google searches. If you are looking for "google contacts free api", then do a google search for that. Once you confirm if you can use their api for free, then look over their documentation or do another search for "google contacts api php" or something along those lines. If the api is available then there is a php sdk for it.
I have not worked with google's contacts api before, but it would be like working with any other api. Find a good SDK/Package that you like, include it in your app, and go to town... What kind of pointers are you looking for?
I can read the entries, but not modify them. When i try to do the PUT request, i get 400 Bad Request. If anyone has any experience with this, i would appreciate the help!