2. TYPO3 Extension API

2.1. Extension Architecture

2.1.1. Introduction

TYPO3 can be extended in nearly any direction without loosing backwards compatibility. The Extension API provides a powerful framework for easily adding, removing, installing and developing such extensions to TYPO3. This is in particular powered by the Extension Manager (EM) inside TYPO3 and the online TYPO3 Extension Repository (TER) found at typo3.org for easy sharing of extensions.

"Extensions" is a term in TYPO3 which covers two other terms, plugins and modules.

A plugin is something that plays a role on the website itself. Eg. a board, guestbook, shop etc. It is normally enclosed in a PHP class and invoked through a USER or USER_INT cObject from TypoScript. A plugin is an extension in the frontend.

A module is a backend application which has its own position in the administration menu. It requires backend login and works inside the framework of the backend. We might also call something a module if it exploits any connectivity of an existing module, that is if it simply adds itself to the function menu of existing modules. A module is an extension in the backend.

Extensions and the Core

Extensions are designed in a way so that extensions can supplement the core seemlessly. This means that a TYPO3 system will appear as "a whole" while actually being composed of the core application and a set of extensions providing various features. This philosophy allows TYPO3 to be developed by many individuals without loosing fine control since each developer will have a special area (typically a system extension) of responsibility which is effectively encapsulated.

So, in one end of the spectrum system extensions makes up what is known as "TYPO3" to the outside world. In the other end, extensions can be 100% project specific and carry only files and functionality related to a single implementation.