Gmail XOAUTH2 Using Google API Client

The official PHPMailer wiki provides an example of using Google's XOAuth2 SMTP authentication using the League OAuth2 client library. This library works fine, but has one drawback to it.

For small projects, using league/oauth2-client library to make api call to Google will do just fine but it comes with many dependencies; ircmaxell/random-lib, guzzlehttp/guzzle, and a few others. On top of this, you must install the league oauth2 google provider library.

But you can still reduce on the number of third party libraries you're using in your project by using the official Google API client with Curl. (Ofcourse if you're using the league OAuth2 library generally in you project for example for authentication, then this would not do you so much good - so stick with that)

Firstly, lets install PHPMailer using composer, remember Google's XOAUTH2 SMTP & IMAP authentication mechanism is only supported starting with PHPMailer 5.2.11. So you must install that or later;
composer require phpmailer/phpmailer ~5.2

So hope you have curl install on your working machine, and enabled for php? If not, you may refer to this great discussion; "How do I install cURL on Windows?" on stackoverflow.

Once you have Curl installed and configured, use composer to install the Google API client library by
composer require google/apiclient 1.*