Use Google Apps for Site Login?

Hey all,
I'm redesigning the infrastructure for a website, and I want to make the login system use the accounts on our Google Apps. How would I be able to do something like this? I'm using PHP for the backend.

Thanks for any help.
-Trey

Is login with Google free?

Google Sign-in is a free service. To use Google sign-in you have to use Google's Firebase authentication service.

How do I find out what apps are connected to my Google Account?

Go to the Security section of your Google Account. Under “Third-party apps with account access,” select Manage third-party access. Select the app or service you want to review.

OAuth is one way, but it might be easier to implement OpenID, which is already provided by Google Apps. See Google Apps domain as OpenID provider