Inside TYPO3

3.2. Extensions

3.2.1. What are extensions

First of all this is only a short description of extensions; For a more detailed description of extension, please see the Extension API section in "TYPO3 Core API".

An "extension" in relation to TYPO3 is a set of files/scripts which can integrate themselves with TYPO3s core through an API an thus seemlessly extend the capabilities of TYPO3.

These are the basic properties of extensions:

1. All files contained within a single directory

2. Easily installed/removed/exchanged

3. Has a unique key (extension key) used for naming of all elements (variables, database tables, fields, classes etc.).

4. Can interact with any part of the system. If not through the available APIs, ultimately (almost) any class in TYPO3 can be extended with full backwards compatibility maintained.

Where are extensions located?

 

Extensions can be installed in three locations:

1. typo3/ext/: Global extensions. A part of the source code directory. Available to all TYPO3 installations sharing the same sourcecode. Is not necessarily available! You can remove or add extensions here and some source distributions will not contain the ext/ directory with global extensions (in which case you will have to add them yourself from TER).

2. typo3/sysext/: System extensions. Just like global extensions: A part of the source code directory. But the system extensions are always distributed with the source code so you can depend on them being there. Further you generally don't need to upgrade system extensions manually as they are upgraded with new source code releases. System extensions carry a special status of being officially endorsed by the TYPO3 system and they are required to match the quality of the core code regarding the standards set out in the TYPO3 Coding Guidelines.

3. typo3conf/ext/: Local extensions: Only available to the local TYPO3 installation. This is the typical location for most extensions which are installed on a per-project basis since the extension is used in only this one case. Also the position for user defined extensions.

What can they change?

Extensions can change practically anything in TYPO3. The concept is very capable since it was created to add limitless power to TYPO3 without having to directly change the core. As such extensions will make it possible for TYPO3 to be a true framework for just any application you can imagine. Installing one set of extensions will make TYPO3 one application - installing another set of extension will make TYPO3 another application. And the core is thus a basic set of modules, an Extension Manager and an API provided for the extensions so they can use core features right away.

Although the basic rule is "anything is possible" this is at least a partial list of features provided by extensions:

1. Addition of database tables and fields to existing tables.

2. Addition of tables with static information

3. Addition of TypoScript static template files or adhoc snippets

4. Addition of backend skins

5. Addition of frontend plugins of any kind

6. Addition of backend modules of any kind

7. Addition of click-menu items (context sensitive menus)

8. Addition of Page and User TSconfig

9. Addition of configuration values

10. Extension of any class in the system

... and of course all kinds of combinations.