This code does not work anymore. And https://google-api-example.herokuapp.com has been terminated.
When this program was first developed, websites did not need to go through an approval process to grant read/write access to password protected Google Docs and sheets. An updating Heroku server caused the Google credentials to be invalidated.
Basically, a PHP site used to use a Google spreadsheet to authorize who could log into a PHP program, and what system rights the user has access to.
For instance, steenhansen1942@gmail.com, can log in and view general information. He can also view the 'Docs Manual' whereas test@gmail.com cannot.
The PHP program could change the values in the Google spreadsheet, so as to delete or change a user's name. But the PHP program would never know any passwords.
Automatically sign in with 'guest_account' of test@gmail.com
https://google-api-example.herokuapp.com/guestSignIn.php
Explanations and links to Google doc and sheet used
https://google-api-example.herokuapp.com/index.html
Sign in with a "real" Google account that exists in the Google sheet of users
https://google-api-example.herokuapp.com/login.php
Sign in via URL
https://google-api-example.herokuapp.com/signInFromGoogle.php?gmail=test@gmail.com
The locked but visible Google sheet https://docs.google.com/spreadsheets/d/1vhce-ziBi6wneqJrxd7Zhyqth8ZLHMJHot1wxgK_T7k/edit#gid=0 that is read and written to.
Sheet/Document/Email/Authorization Classes
php-google-api-example/Google_Api/
To install composer libraries
php-google-api-example/ composer install
To enable Google credentials
Copy google-sample files to google-account directory
To test
php-google-api-example/ php tests/call_tests.php