You can also look up for these other topics:
Framework
Coding Standards
Boilerplate
CMS
Tool
Security
Hacktoberfest
Single file PHP script that adds a REST API to a SQL database
This Bundle provides various tools to rapidly develop RESTful API's with Symfony
:date: Easy!Appointments - Self Hosted Appointment Scheduler
Exposes Advanced Custom Fields Endpoints in the WordPress REST API
Exemplary real world backend API built with Laravel