Plesk Extensions API stubs is a set of files with all classes definitions and methods prototypes, but without implementation of these methods. You can use API stubs for quicker development.
The first step is to checkout the API stubs repository. It’s located at https://github.com/plesk/pm-api-stubs. You need to check out stubs to a separate directory (do not mix with extension’s source code). Later you can use these stubs for all extension projects you’ve started. If you have no Git client for checkout, download a zip archive and unpack it – https://github.com/plesk/pm-api-stubs/archive/master.zip.
Note: The repository https://github.com/plesk/pm-api-stubs contains API stubs for latest released Plesk version. API stubs for older versions are placed in separate branches.
Next step is to configure an IDE (for example, PhpStorm) to include the stubs as an external library. Please see the details in the Plesk developers blog in the article: Extensions: IDE and APIs Hinting.
As a result, if you open a file with source code and start typing the code, completion of API methods and classes will start working and you do not need to to check API documentation at every step.