Lorna Mitchell has a new post to her blog today showing how to use the functionality provided by the pecl_http extension to make an OAuth2 connection to Google.

I've written about Google and OAuth before, but that was OAuth v1.0, and they are introducing OAuth2 for their newer APIs; in this example I was identifying myself in order to use the Google Plus API. [...] OAuth 2 doesn't need an extension or any particular library as it doesn't have the signing component that OAuth 1 had, and OAuth 2 also has fewer round trips. It does require SSL however, because the requests are in the clear.

She includes some code snippets with an example of a connection - making a request to the remote HTTPS resource, adding some parameters to the URL (including the response type, your client ID and a redirect url). The response then contains the "code" value you'll need to make the second request to fetch the access token you'll need on future requests. You can find out more about the interface she's accessing in these docs about the Google Plus API.