Built in coordinate systems
In ISO 19111 terminology, what most people refer to as a Coordinate System is actually called a Coordinate Reference System (or CRS for short), with the term Coordinate System being used to refer to the axes used. PHPCoord’s actual code uses the ISO naming system, although this documentation uses the more common version.
A constant has been defined for each and every built in CRS, with supporting docblocks that match the contents of the following pages. These should be searchable/available for autocomplete from inside your favourite IDE.