Find YouTube username with YouTube / GooglePlus API - php

Question

I'm fairly new to Stack Overflow, so if I am doing something wrong please tell me;
I have setup my website with the Google+ API - to log in, people click on a "Login With Google+" button and log in.
That's all fine and good, but I was wondering if I can integrate this login process with the YouTube API, so that when the user clicks the button to log in, I can also gather YouTube information.
I've already setup the YouTube Analytics and Data APIs on the Google API Console, I just need a code snippet to store the YouTube Username.
Expected Behavior:
User with URL 'youtube.com/some-user' logs in with Google+
Google+ Authentication Key is fed into some script
Script connects to YouTube API and returns username (eg $youtube_user = 'some-user')
My website takes this information and continues with the script

Answer 1

You can do this using the YouTube API. What you need to do is add an additional scope to your sign-in button. I would recommend you use the following scope:
https://www.googleapis.com/auth/youtube.readonly
Do this because you will not be managing anyone's YouTube account, just seeing their personal data (e.g. to pull in links or embed code for their YouTube videos).
Next, you need to perform an API call to YouTube using a client library. For experimentation, you can use the API explorer. To see your own information, authorize the request, set the part to "snippet" (no quotes) and then set mine to true.
Download the "youtube", "v3" static class from the PHP Google API client library, then make a call passing snippet as the part you want and "mine" set to true.
For listing the current user's videos, there is an example that ships with the PHP client library. The following change would list the user's activities:
// Exchange the OAuth 2.0 authorization code for user credentials.
$client->authenticate($code);
$token = json_decode($client->getAccessToken());
...
$activities = $youtube->activities->listActivities('snippet', array(
'mine' => 'true',
));
foreach ($activities['items'] as $activity) {
error_log(serialize($activity));
}

Answer 2

class's answer is spot on in terms of getting the OAuth 2 credentials to make the request. In terms of actual PHP code to work off of, you can use this example as a starting point (which uses this client library), and modify line 40 to read
$channelsResponse = $youtube->channels->listChannels('snippet', array(
'mine' => 'true',
));
And then you can access the channel's title via $channelsResponse['items'][0]['snippet']['title']. Note that it's possible to link a Google+ account and a YouTube channel, and if that's done, the channel's title will be equal to the Google+ account's display name. If this is an unlinked YouTube channel, then the legacy username of the channel will be returned instead.