Plugins are developed in java using IntelliJ IDEA (Community Edition is OK). You will also need a PhpStorm copy to develop against. PsiViewer plugin will also be most useful.
- IntelliJ IDEA Plugin Development
- IntelliJ Platform plugin development Tutorials
- Webinar: Live Coding a Plugin from Scratch. Part 1. Live Coding a Plugin from Scratch. Part 2.
- Plugin Development FAQ
- Forum about OpenAPI