If you are a PHP developer new to Moodle development, a good starting point is the free self-paced course Moodle development environment on Moodle Academy.
Inspired by the original Moodle Plugin Development Basics, the course Moodle development environment is the first course in an upcoming program of courses that step you through how to start developing with Moodle. You'll learn about the Moodle developer community and set up a Moodle development environment in order to start developing plugins.
The course has an estimate learning time of 3 hours, and you're welcome to create an account on Moodle Academy and explore the course. A badge is available if you complete the required activities