Has anybody implemented this and had any success? I keep getting “401. That?s an error. Error: invalid_client” no matter what I try. I have changed the name of the php file, various registrations in the Google API. This is possibly something simple that I am missing but I am not seeing it. Just hoping somebody else has been here, done this and worn the t-shirt - and survived. 
Anybody, anybody?
You should be able to code it in standard php using the api.
https://developers.google.com/accounts/
https://developers.google.com/+/api/oauth or https://developers.google.com/+/web/signin/
https://developers.google.com/+/web/signin/server-side-flow
its a bit techy but with some googling you should be able to find several solutions in no time.
There are some examples on the web… Be sure to use only the newest oauth 2.
Interesting statement about the newest version of oauth2. I am not having any joy with connecting using the directions from ScriptCase. I get further with Facebook authentication but still not working. Just a different error but further along the process. I do not want to use the Facebook authentication. I just wanted to see if it would expose why I am having problems with the G+ authentication. It didn’t. 
Honestly we just made a simple login screen with customised authentication code (in php) behind it. Basic and simple to implement. In our case we use ldap but it would not really make a difference since all you finally get back is OK or NOT OK in one way or another.
So you could even build a library or better yet be super laze and search on the internet.
Within 5 seconds I found http://hybridauth.sourceforge.net/
Talk about a super easy implementation…
Thank you very much for the suggestion. My reservation is that Scriptcase claims to provide the facility and as such I should not need to look for a solution outside of Scriptcase. Now I say “claims” with all due respect. It may be something I am doing wrong but I have spent a lot of time searching for answers and trying different things to get the Scriptcase offering to work. I was hoping that somebody else had been down this road. Interestingly, the Scripcase demo also does not seem to work.
I didn’t answered before because I didn’t try it, and very busy right now to try