There are several ways you can work with DeskPRO as a developer:

The DeskPRO REST web API - comes with its own PHP SDK.

DeskPRO Widgets: an easy way to add simple widgets to the agent interface using just Javascript and HTML: useful if you want to pull data from another system and display it on tickets, for example

DeskPRO Apps: you can develop Apps like the ones you can install from Admin > Apps. Enable more elaborate integrations or interface changes.