ATTENTION: THIS IS WORK IN PROGRESS AND NOT TO BE USED UNTIL THIS MESSAGE DISAPPEARS!
PHP/SAP defines a layer of abstraction between actual SAP remote function calls and your PHP-Code. As to why this project has been created and which goals it wants to achieve, you can read the motivation.
The interfaces define a common denominator on how to configure a connection to SAP, prepare a SAP remote function call, and invoke a SAP remote function call.
The common classes and exceptions add logic that is not specific to the underlying PHP module.
The module specific implementations contain code that is specific to the underlying PHP-module.