I receive confirmation from users in my gmail account. I want to read them with a server side php script to display confirmation on my website. I got OAuth 2.0 client IDs and a service key from Google APis. How do I use these keys with the specific gmail account to get the emails? I cannot find any simple documentation.
You can not use a service account to access a normal users Gmail account. This is because there is no way to preauthorize it.
What you need to do is have the users authenticate your application using Oauth2 and get a refresh token for each user. Then you will be able to use the refresh token in your server sided script to request a new access token which you can use to read there gmails. The Google PHP client library team has created some examples for using their library which might help you. Google-api-php-client
Side note: If the users you are speaking of are part of a Google domains account then you can do it by giving service account permissions to access the users emails.