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.