Manual:Coding conventions

This page describes the coding conventions used within the MediaWiki codebase and extensions which are intended for use on Wikimedia websites, including appropriate naming conventions.

All languages
Some general conventions apply to all MediaWiki code, whatever language it is written in.

Tab size
Lines should be indented with a single tab character per indenting level. You should make no assumptions about the number of spaces per tab. Most MediaWiki developers find 4 spaces per tab to be best for readability, but many systems are configured to use 8 spaces per tab and some developers might use 2 spaces per tab.

Newlines
All text files should be checked in to Subversion with  set to "native". This is necessary to prevent corruption by certain Windows-based text editors.

You can ask subversion to automatically set this property on a given list of file type. This is done with subversion auto-props and will save you the hassle to manually set the property when adding new files.

You might want to read the English Wikipedia article about newline.

Encoding
All text files must be encoded with UTF-8 without byte order marks.

Do not use Microsoft Notepad to edit files. Notepad always inserts BOM. BOM will stop PHP files from working since it is a special character inserted at the very top of the file and will be outputted by the web browser to the client.

In short, make sure your editor supports UTF-8 without BOM.

General style
MediaWiki's indenting style is similar to the so-called "One True Brace Style". Braces are placed on the same line as the start of the function, conditional, loop, etc.

Multi-line statements are written with the second and subsequent lines being indented by one extra level:

Use indenting and line breaks to clarify the logical structure of your code. Expressions which nest multiple levels of parentheses or similar structures may begin a new indenting level with each nesting level:

There are some exceptions, such as switch statements, where the indentation of the cases are optional, so both of the below are fine.

Vertical alignment
Avoid vertical alignment. It tends to create diffs which are hard to interpret, since the width allowed for the left column constantly has to be increased as more items are added.

When needed, create mid-line vertical alignment with spaces. For instance this:

Is achieved as follows with spaces rendered as dots:

$namespaceNames·=·array(  →    NS_MEDIA············=>·'Media',   →    NS_SPECIAL··········=>·'Special',   →    NS_MAIN·············=>·'',

Line continuation
Lines should be broken at between 80 and 100 columns. There are some rare exceptions to this. Functions which take lots of parameters are not exceptions.

The operator separating the two lines may be placed on either the following line or the preceding line. An operator placed on the following line is more visible and so is more often used when the author wants to draw attention to it:

An operator placed on the preceding line is less visible, and is used for more common types of continuation such as concatenation and comma:

When continuing "if" statements, a switch to Allman-style braces makes the separation between the condition and the body clear:

Opinions differ on the amount of indentation that should be used for the conditional part. Using an amount of indentation different to that used by the body makes it more clear that the conditional part is not the body, but this is not universally observed.

Continuation of conditionals and very long expressions tend to be ugly whichever way you do them. So it's sometimes best to break them up by means of temporary variables.

Spaces
MediaWiki favors a heavily-spaced style for optimum readability.

Put spaces on either side of binary operators, for example:

not like this:

Put spaces next to parentheses on the inside, except where the parentheses are empty. Do not put a space following a function name.

Opinions differ as to whether control structures, , and  should be followed by a space; the following two styles are acceptable:

Single-line comments should have a space between the or  and the comment text.

To help developers fix code with an inadequately spacey style, a tool called stylize.php has been created, which uses PHP's tokenizer extension to add spaces at the relevant places.

Braceless control structures
Do not write "blocks" as a single-line:

They reduce the readability of the code by moving important statements away from the left margin, where the reader is looking for them. Remember that making code shorter doesn't make it simpler. The goal of coding style is to communicate effectively with humans, not to fit computer-readable text into a small space.

Most MediaWiki developers favor fully-braced control structures:

This avoids a common logic error, which is especially prevalent when the developer is using a text editor which does not have a "smart indenting" feature. The error occurs when a single-line block is later extended to two lines:

Later changed to:

This has the potential to create subtle bugs.

emacs style
In emacs, using  from nXHTML mode, you can set up a MediaWiki minor mode in your .emacs file:

The above  function will check your path when  is invoked to see if it contains “mw” or “mediawiki” and set the buffer to use the   minor mode for editing MediaWiki source. You will know that the buffer is using  because you'll see something like “PHP MW” or “PHP/lw MW” in the mode line.

File naming
Files which contain server-side code should be named in UpperCamelCase. This is also our naming convention for extensions. Name the file after the most important class it contains; most files will contain only one class, or a base class and a number of descendants. For instance,  contains only the   class;   contains the base class , but also the related class   and its descendants.

Name 'access point' files, such as SQL, and PHP entry points such as  and , in lowercase. Maintenance scripts are generally in lowerCamelCase, although this varies somewhat. Files intended for the site administrator, such as readmes, licenses and changelogs, are usually in UPPERCASE.

Never include spaces in file names or directories, and never use non-ASCII characters. For lowercase titles, hyphens are preferred to underscores.

For JS and CSS files loaded via ResourceLoader, file naming should match module naming. For example:
 * module  has files   and
 * module  has file

JS and CSS files for extensions usually use a name like, for instance:

This keeps it easy to find things, even if you divide up a module into smaller files for editing convenience and then bundle them together into a single module.

Groups of modules should have their files also grouped in directories. For example, there are several modules related to " ". All those modules start with " " and are stored in the  directory.

Assignment expressions
Using assignment as an expression is surprising to the reader and looks like an error. Do not write code like this:

Space is cheap, and you're a fast typist, so instead use:

Using assignment in a clause used to be legitimate, for iteration:

This is unnecessary in new code; instead use:

Ternary operator
The ternary operator can be used profitably if the expressions are very short and obvious:

But if you're considering a multi-line expression with a ternary operator, please consider using an block instead. Remember, disk space is cheap, code readability is everything, "if" is English and  is not.

5.3 shorthand
Since we still support PHP 5.2.x, use of the shorthand ternary operator introduced in PHP 5.3 is not allowed.

String literals
For simple string literals, single quotes are slightly faster for PHP to parse than double quotes. Perhaps more importantly, they are easier to type, since you don't have to press shift. For these reasons, single quotes are preferred in cases where they are equivalent to double quotes.

However, do not be afraid of using PHP's double-quoted string interpolation feature: This has slightly better performance characteristics than the equivalent using the concatenation (dot) operator, and it looks nicer too.

Heredoc-style strings are sometimes useful:

Some authors like to use END as the ending token, which is also the name of a PHP function. This leads to IRC conversations like the following:

	vim also has ridiculously good syntax highlighting. 	it breaks when you write <<	TimStarling, but if you write <<	I have to keep changing it to ENDS so it looks like a string again 	fix the bug in vim then! 	brion-codereview: have you ever edited a vim syntax script file? 	hehehe 	http://tstarling.com/stuff/php.vim 	that's half of it... 	here's the other half: http://tstarling.com/stuff/php-syntax.vim 	1300 lines of sparsely-commented code in a vim-specific language 	which turns out to depend for its operation on all kinds of subtle inter-pass effects TimStarling: it looks like some franken-basic language.

Functions and parameters
Avoid passing huge numbers of parameters to functions or constructors: It quickly becomes impossible to remember the order of parameters, and you will inevitably end up having to hardcode all the defaults in callers just to customise a parameter at the end of the list. If you are tempted to code a function like this, consider passing an associative array of named parameters instead.

In general, using boolean parameters is discouraged in functions. In, without looking up the documentation for , it is impossible to know what those parameters are meant to indicate. Much better is to either use class constants, and make a generic flag parameter: Or to make your function accept an array of named parameters:

Try not to repurpose variables over the course of a function, and avoid modifying the parameters passed to a function (unless they're passed by reference and that's the whole point of the function, obviously).

C borrowings
The PHP language was designed by people who love C and wanted to bring souvenirs from that language into PHP. But PHP has some important differences from C.

In C, constants are implemented as preprocessor macros and are fast. In PHP, they are implemented by doing a runtime hashtable lookup for the constant name, and are slower than just using a string literal. In most places where you would use an enum or enum-like set of macros in C, you can use string literals in PHP.

PHP has three special literals:, and. Homesick C developers write as  because they want to believe that it is a macro defined as. This is not necessary.

Use not. They have subtly different meanings: And the latter has poorer performance.

PHP pitfalls

 * Understand and read the documentation for  and  . Use them only when appropriate.
 * is inverted conversion to boolean with error suppression. Only use it when you really want to suppress errors. Otherwise just use . Do not use it to test if an array is empty, unless you simultaneously want to check if the variable is unset.
 * Do not use to test for .  Using  in this situation could introduce errors by hiding mis-spelled variable names. Instead, use
 * Study the rules for conversion to boolean. Be careful when converting strings to boolean.
 * Be careful with double-equals comparison operators. Triple-equals is often more intuitive.
 * is true
 * is true
 * is false
 * Array plus does not renumber the keys of numerically-indexed arrays, so . If you want keys to be renumbered, use array_merge:
 * Make sure you have  set to  for PHP 5. This will notify you of undefined variables and other subtle gotchas that stock PHP will ignore. See also Manual:How to debug.
 * When working in a pure PHP environment, remove any trailing  tags. These tags often cause issues with trailing white-space and "headers already sent" error messages (cf. 17642 and http://news.php.net/php.general/280796).
 * Do not use the 'goto' syntax introduced in 5.3. PHP may have introduced the feature, but that does not mean we should use it.

Integration
There are a few pieces of code in the MediaWiki codebase which are intended to be standalone and easily portable to other applications; examples include the UTF normalisation in  and the libraries in. Apart from these, code should be integrated into the rest of the MediaWiki environment, and should allow other areas of the codebase to integrate with it in return.

Global objects
Do not access the PHP superglobals, , etc, directly; use instead; there are various functions depending on what type of value you want. You can get a from the nearest, or if absolutely necessary. Equally, do not access directly; use  if you want to get the IP address of the current user.

Classes
Encapsulate your code in an object-oriented class, or add functionality to existing classes; do not add new global functions or variables. Try to be mindful of the distinction between 'backend' classes, which represent entities in the database (eg,  ,  , etc), and 'frontend' classes, which represent pages or interfaces visible to the user ( ,  ,  , etc. Even if your code is not obviously object-oriented, you can put it in a static class (eg  ).

As a holdover from PHP 4's lack of private class members and methods, older code will be marked with comments such as to indicate the intention; respect this as if it were enforced by the interpreter.

Mark new code with proper visibility modifiers, including if appropriate, but do not add visibility to existing code without first checking, testing and refactoring as required. It's generally a good idea to avoid visibility changes unless you're making changes to the function which would break old uses of it anyway.

Error handling
Don't suppress errors with PHP's operator, for any reason ever. It's broken when  is enabled and it causes an unlogged, unexplained error if there is a fatal, which is hard to support. Use  and   instead. The checkSyntax.php maintenance script can check for this error for you.

When your code encounters a sudden error, you should throw a MWException (or an appropriate subclass) rather than using PHP's trigger_error. The exception handler will display this as nicely as possible to the end user and wiki administrator, and also provides a stack trace to developers.

PHP Naming
Use lowerCamelCase when naming functions or variables. For example:  Use UpperCamelCase when naming classes:. Use uppercase with underscores for global and class constants:,. Other variables are usually lowercase or lowerCamelCase; avoid using underscores in variable names.

There are also some prefixes used in different places:

Functions

 * (wiki functions) – top-level functions, e.g.

Verb phrases are preferred: use instead of.

Variables

 * – global variables, e.g., . Always use this for new globals, so that it's easy to spot missing "" declarations. In extensions, the extension name should be used as a namespace delimiter. For example, , not.

It is common to work with an instance of the  class; we have a naming convention for these which helps keep track of the nature of the server to which we are connected. This is of particular importance in replicated environments, such as Wikimedia and other large wikis; in development environments there is usually no difference between the two types, which can conceal subtle errors.


 * – a  object for writing (a master connection)
 * – a  object for non-concurrency-sensitive reading (this may be a read-only slave, slightly behind master state, so don't ever try to write to the database with it, or get an "authoritative" answer to important queries like permissions and block status)

The following may be seen in old code but are discouraged in new code:


 * – Session variables, e.g.
 * – Cookie variables, e.g.
 * – Post variables (submitted via form fields), e.g.
 * – object member variables: . This is discouraged in new code, but try to stay consistent within a class.

Inline
The Doxygen documentation style is used (it is very similar to PHPDoc for the subset that we use). A code documentation example: giving a description of a function or method, the parameters it takes (using ), and what the function returns (using  ), or the   or   tags.

Use  rather than   as the escape character (i.e. use  ) – both styles work in Doxygen, but for backwards and future compatibility MediaWiki uses has chosen the   to begin the comments, instead of the Qt-style formating.

General format for parameters is such:. Multiple types can be listed by separating with a pipe character.

PHPDoc was used at the very beginning but got replaced with Doxygen for performance reason. We should probably drop PHPDoc compatibility.

There is a bug in Doxygen that affect MediaWiki's documentation:
 * Bug 626105 - @var in php is not documented
 * should be usable to document class members' type, but doesn't work for now.

folder
Some elements of MediaWiki are documented in the folder. For instance, if you add a new hook, you should update docs/hooks.txt with the name of the hook, a description of what the hook does, and the parameters used by the hook.

Release notes
All significant changes to the core software which might affect wiki users, server administrators, or extension authors, must be documented in the  file. This file is refreshed on every release (with the past content going into ) and is generally divided into three sections:


 * Configuration changes is the place to put changes to accepted default behaviour, backwards-incompatible changes, or other things which need a server administrator to look at and decide "is this change right for my wiki?". Try to include a brief explanation of how the previous functionality can be recovered if desired.
 * Bug fixes is the place to note changes which fix behaviour which is accepted to be problematic or undesirable. These will often be issues reported in bugzilla, but needn't necessarily.
 * New features is, unsurprisingly, to note the addition of new functionality.

In all cases, if your change is in response to an issue reported in bugzilla, include the bug reference at the start of the entry. New entries are added in chronological order at the end of the section.

Messages
When creating a new message, use hyphens (-) where possible instead of CamelCase or rails_underscored_variables. So for example,  is a good name, while   and   are not.

If the message is going to be used as a label which can have a colon after it, don't hardcode the colon; instead, put the colon inside the message text. Some languages (such as French which require a space before) need to handle colons in a different way, which is impossible if the colon is hardcoded.

Try to use message keys "whole" in code, rather than building them on the fly; as this makes it easier to search for them in the codebase. For instance, is bad because a search for  will not find this use of the message key. Instead do:

Module naming
Module names should match the main definition of the scripts they load. For example a module defining the the  object is named "mediawiki.util" and the module for the   object constructor is named "mediawiki.Title".

See also:
 * 
 *