These technologies make it easy to implement.
Let's implement Twitter login with PHP https://dotinstall.com/lessons/tw_connect_php_v3#!lessons
Let's create a web service to log in with Google https://dotinstall.com/lessons/google_connect_php_v2
Firebase https://firebase.google.com/?hl=ja
[Rails] Implement login function https://qiita.com/d0ne1s/items/7c4d2be3f53e34a9dec7
Simple login function made with php https://qiita.com/qwertyuiopngsdfg/items/597da67387723a5aedad
Let's implement login function with PHP 5.6 https://dotinstall.com/lessons/sns_php_v3
Explaining how to send an email with PHP [Easy to understand even for the first time] https://techplay.jp/column/550
I made an image uploader with Python (CGI) https://masublog.net/python/pythoncgiimageuploader/
Batch resizing (enlarging / reducing) images with Python and Pillow https://note.nkmk.me/python-pillow-image-resize/