Identify Google user from third party web application

I'm developing an Android application that connects to a web service written in PHP and saves/retrieves some user information.

I need a way to securely identify users of that application on the web server. I can easily use AccountManager to retrieve user email and send it to the web server, but what is the simplest way to authenticate it?

My web service doesn't use any Google services/APIs. The only thing that I need is to confirm (on web server side) that user@gmail.com is really user@gmail.com.