You need the gmail API for that, but you can integrate the authentication:
Basically, when you ask for authentication you not only ask for AdWords access but also mail access. The user will see a consent form stating that your software will access their AdWords accounts(s) and also their mail. Once authenticated you can use the refreshtoken/accesstoken for both AdWords API and gmail API.
Note I don't use PHP myself so I don't know how this works out in PHP. But you should get the idea.
There is a downside to this approach: If you store refreshtokens (for instance, you use them for batch processing in the night), do realize that they become invalid when the user changes his password. This happens to all refreshtokens having access to gmail API. (In other words, a refreshtoken with only access to AdWords API will survive a password change of the authenticated user. But a refreshtoken that also has access to gmail API will not survive a password change.) Again, this is only relevant if you store refreshtokens, for instance for batch processing.
Hope this helps.