Answer by jlmcdonald for Youtube API Retrieving Channel ID after authentication

Since you're using PHP, I'm assuming you're using the PHP-based gapi-client? Here's the call to make:

$listResponse = $youtube->channels->listChannels('id', array('mine' => 'true',))

The 'id' part returns the ID of the channel, and by passing the parameter "mine" as true, you're telling the API to return the data for the authenticated user.

If you're using a different client or rolled your own, you can also just hit this endpoint (passing the oAuth access token you have in the header or as another parameter to the request):

GET https://www.googleapis.com/youtube/v3/channels?part=id&mine=true&key={YOUR_API_KEY}