This page is obsolete. It is kept for historical interest only. It may document extensions or features that are obsolete and/or no longer supported. Do not rely on the information here being up-to-date.
The file docs/design.txt in the MediaWiki core repository was created in 2003 to describe the basic architecture of MediaWiki at the time, and remained mostly unchanged since then, and thus has grown inaccurate and misleading. It is reproduced here for posterity, before removing it from the code repository.
design.txt This is a brief overview of the new design. More thorough and up-to-date information is available on the documentation wiki at https://www.mediawiki.org/ Primary classes: User Encapsulates the state of the user viewing/using the site. Can be queried for things like the user's settings, name, etc. Handles the details of getting and saving to the "user" table of the database, and dealing with sessions and cookies. OutputPage Encapsulates the entire HTML page that will be sent in response to any server request. It is used by calling its functions to add text, headers, etc., in any order, and then calling output() to send it all. It could be easily changed to send incrementally if that becomes useful, but I prefer the flexibility. This should also do the output encoding. The system allocates a global one in $wgOut. Title Represents the title of an article, and does all the work of translating among various forms such as plain text, URL, database key, etc. For convenience, and for historical reasons, it also represents a few features of articles that don't involve their text, such as access rights. See also title.txt. Article Encapsulates access to the "page" table of the database. The object represents a an article, and maintains state such as text (in Wikitext format), flags, etc. Revision Encapsulates individual page revision data and access to the revision/text/blobs storage system. Higher-level code should never touch text storage directly; this class mediates it. Skin Encapsulates a "look and feel" for the wiki. All of the functions that render HTML, and make choices about how to render it, are here, and are called from various other places when needed (most notably, OutputPage::addWikiText()). The StandardSkin object is a complete implementation, and is meant to be subclassed with other skins that may override some of its functions. The User object contains a reference to a skin (according to that user's preference), and so rather than having a global skin object we just rely on the global User and get the skin with $wgUser->getSkin(). See also skin.txt. Language Represents the language used for incidental text, and also has some character encoding functions and other locale stuff. The current user interface language is instantiated as $wgLang, and the local content language as $wgContLang; be sure to use the *correct* language object depending upon the circumstances. See also language.txt. Parser Class used to transform wikitext to html. LinkCache Keeps information on existence of articles. See linkcache.txt. Naming/coding conventions: These are meant to be descriptive, not dictatorial; I won't presume to tell you how to program, I'm just describing the methods I chose to use for myself. If you do choose to follow these guidelines, it will probably be easier for you to collaborate with others on the project, but if you want to contribute without bothering, by all means do so (and don't be surprised if I reformat your code). - I have the code indented with tabs to save file size and so that users can set their tab stops to any depth they like. I use 4-space tab stops, which work well. I also use K&R brace matching style. I know that's a religious issue for some, so if you want to use a style that puts opening braces on the next line, that's OK too, but please don't use a style where closing braces don't align with either the opening brace on its own line or the statement that opened the block--that's confusing as hell. - Certain functions and class members are marked with /* private */, rather than being marked as such. This is a hold-over from PHP 4, which didn't support proper visibilities. You should not access things marked in this manner outside the class/inheritance line as this code is subjected to be updated in a manner that enforces this at some time in the near future, and things will break. New code should use the standard method of setting visibilities as normal. - Globals are particularly evil in PHP; it sets a lot of them automatically from cookies, query strings, and such, leading to namespace conflicts; when a variable name is used in a function, it is silently declared as a new local masking the global, so you'll get weird error because you forgot the global declaration; lack of static class member variables means you have to use globals for them, etc. Evil, evil. I think I've managed to pare down the number of globals we use to a scant few dozen or so, and I've prefixed them all with "wg" so you can spot errors better (odds are, if you see a "wg" variable being used in a function that doesn't declare it global, that's probably an error). Other conventions: Top-level functions are wfFuncname(), names of session variables are wsName, cookies wcName, and form field values wpName ("p" for "POST").
History and attribution:
2014-01-28 Ladsgroup http://www.mediawiki.org --> https://www.mediawiki.org 2012-01-29 Jeroen De Dauw this is no longer a guideline afaik 2009-08-16 Alexandre Emsenhuber * maintenance.txt: the execute() method must be public to match Maintenance's one * design.txt: whitespaces fix 2008-07-17 Alexandre Emsenhuber Doc tweaks: * Moved doc about primary scripts to scripts.txt * Lines ending to 80 chars 2007-06-26 Rob Church Some updates; regarding method/member visibilities, OutputPage is not Parser, and Language objects 2007-01-08 Brion Vibber doc updates 2005-08-23 Antoine Musso fix 3175 2005-04-12 Ævar Arnfjörð Bjarmason * Changed Wikipedia to MediaWiki. 2005-04-12 Brion Vibber Change .doc extension to .txt so people stop asking why we have Word documents. WE DONT THEY ARE TEXT!!!!111eleven 2004-12-02 Evan Prodromou Further explanation as to why to avoid CVS keywords. 2004-11-29 Evan Prodromou Removed CVS keywords from files, to make merging between branches easier. Interpolated keywords cause lots of conflicts and headaches at merge time for older (<1.12.x) CVS versions. 2004-07-25 Arne Heizmann sp 2004-02-28 Erik Moeller using index.php and redirect.php instead of wiki.phtml and redirect.phtml keeping phtml stubs around for compatibility and CVS history 2003-04-14 Lee Daniel Crocker Initial revision