This page is the top-level page describing Moodle's coding guidelines. It's the place to start if you want to know how to write code for Moodle.
Moodle tries to run on the widest possible range of platforms, for the widest possible number of people, while remaining easy to install, use, upgrade and integrate with other systems.
For more about this, see Moodle architecture.
Moodle has a general philosophy of modularity. There are nearly 30 different standard types of plugins and even more sub-plugin types, however all of these plugin types work the same way. Blocks and activities are the only small exceptions.
Consistent coding style is important in any development project, and particularly so when many developers are involved. A standard style helps to ensure that the code is easier to read and understand, which helps overall quality.
Writing your code in this way is an important step to having your code accepted by the Moodle community.
Our Moodle coding style document explains this standard.
Security is about protecting the interests and data of all our users. Moodle may not be banking software, but it is still protecting a lot of sensitive and important data such as private discussions and grades from outside eyes (or student hackers!) as well as protecting our users from spammers and other internet predators.
It's also a script running on people's servers, so Moodle needs to be a responsible Internet citizen and not introduce vulnerabilities that could allow crackers to gain unlawful access to the server it runs on.
Any single script (in Moodle core or a third party module) can introduce a vulnerability to thousands of sites, so it's important that all developers strictly follow our Moodle security guidelines.
XHTML and CSS
It's important that Moodle produces strict, well-formed HTML 5 code (preferably backwards compatible with XHTML 1.1 if possible), compliant with all common accessibility guidelines (such as W3C WAG 2.0, ARIA).
CSS should be used for layout. Moodle comes with several themes installed. Beginning with version 2.7, only the 'Clean' theme comes in the base Moodle code. The 'standard' theme, which should be a plain theme suitable to act as a building block for other themes. That should contain the minimal styling to make Moodle look OK and be functional. Then Moodle comes with several other default themes that look good and demonstrate various techniques for building themes.
This helps consistency across browsers in a nicely-degrading way (especially those using non-visual or mobile browsers), as well as improving life for theme designers.
Moodle works in over 84 languages because we pay great attention to keeping the language strings and locale information separate from the code, in language packs.
The default language for all code, comments and documentation, however, is English (AU).
Full details: String API
Moodle should work well for the widest possible range of people.
See Moodle Accessibility for more information.
See Interface Guidelines (under construction)
The load any Moodle site can cope with will, of course, depend on the server and network hardware that it is running on. However there are some features (intended especially for developers) that are discouraged on production sites for performance reasons.
The most important property is scalability, so a small increase in the number of users, courses, activities in a course, and so on, only causes a correspondingly small increase in server load.
For more information and advice, see Performance and scalability.
Moodle has a powerful database abstraction layer that we wrote ourselves, called XMLDB. This lets the same Moodle code work on MySQL/MariaDB, PostgreSQL, MS SQL Server and Oracle. There are know issues when using Oracle, it is not fully supported and is not recommended for production sites.
Overview: Moodle Database guidelines
Moodle allows inter-module communication via events. Modules can trigger specific events and other modules can choose to handle/observe those events.
Unit testing is not simply a technique but a philosophy of software development.
The idea is to create automatable tests for each bit of functionality that you are developing (at the same time you are developing it). This not only helps everyone later test that the software works, but helps the development itself, because it forces you to work in a modular way with very clearly defined structures and goals.
Moodle uses a framework called PHPUnit that makes writing unit tests fairly simple.
See PHPUnit for more information.
PHPUnit covers mostly the internal implementation of functions and classes, the user interaction testing can be automated using the Behat framework.
See Acceptance testing for more information.
Third Party Libraries
Moodle has a standard way to include third party libraries in your code. See https://docs.moodle.org/dev/Third_Party_Libraries
Please note that Moodle coding style and design is pretty unique, it is not compatible with PEAR coding standards or any other common PHP standards.