Because PHP is an interpreted language, the Moodle code is stored as source code files on the web server. When a particular file is requested on the server, the PHP interpreter parses the code on the fly, and the resulting output is sent out via the web server software. As mentioned earlier, the "M" in Moodle stands for "Modular", and its directory structure reflects that. Each top-level folder represents a major component of Moodle. Many of the main components support plugin modules. Each plugin has its own folder inside the component's folder. In some cases, modules will also have support for additional plugins. An example of this is the quiz activity module, which supports modular question types. From the end user point of view, modules are installed by copying the module into the appropriate folder location on the server. Moodle detects the new module the next time that an administrator logs into the system, locates the module's SQL code, runs it, and finally displays the results.

Upgrades work in much the same way with Moodle, tracking the database version and automatically upgrading the database as needed. All of this easy interface for end users comes at the cost of some elbow grease for the developer.

The following screenshot is a directory listing of a recent Moodle 1.9 installation:

We will not cover all of the directories in the main folder at this time. However, we will explore the functions of some of the important folders used by developers who make modifications to Moodle. Moodle uses a simple nomenclature for modules, where all of the modules are enclosed in their own folder, and the name of the folder is the name that Moodle displays in its interface when referring to the module.

admin:

This folder stores the PHP files that control the administrative user's interface. They also contain the cron.php file, which is run as a batch process to perform system maintenance tasks such as message delivery and course backups. We will often hook into the cron.php process to perform batch operations.

auth:

The auth folder contains all of the authentication modules for Moodle. Each module will have its own directory in this area. Authentication modules control the creation of users, user profile data, and user access to the system. Authentication modules are great for automating system administration, and as a result are a common customization project.

backup:

This folder contains the core course backup facilities for the system. These are not system-wide backup facilities but functions for the backup, restore, and import of courses. Each individual course module is responsible for its own backup code and makes use of these functions as needed. Each module is self-contained, allowing us, as developers, to add modules cleanly to Moodle without having to modify the core code.

blocks:

blocks are used to display boxes of information in either the right-hand side or left-hand side column of the Moodle page. This is one of the simplest module types to make, and also tend to work across multiple versions of Moodle with little or no modifications.

course:

This component of Moodle has obvious importance, given that Moodle is organized around courses. As developers, we are most likely to modify or add course formats and reports. Custom course formats can be used to change the layout of courses.

enrol:

The enrol folder contains all of the enrollment modules for Moodle. Enrollment modules control the creation and management of course-level role assignments (enrollments). Enrollment modules are another key automation hook.

files:

The files component allows Moodle to incorporate files into the system. This includes file uploads, access control, and the viewing of files. files will see a major rewrite in Moodle 2.0. Moodle 2.0 will allow storing and using files in external file repositories such as Alfresco, Box.net, and Google Docs.

filter:

The Moodle filtering system is a text/regular expression-based search-and-replace facility. The filter system is fed user-entered content from the database during page creation. Filters match and modify the page before it is displayed. For example, there is a math filter that supports auto conversion of TEX markup language to equation graphics. The Multimedia Plugins filter finds references to common media types and wraps the text in the appropriate embed and/or object tags, in order to automatically embed the media, along with player controls, into the page. This is a very powerful capability. However, it needs to be carefully developed, with performance implications in mind.

lang:

The lang folder stores the core system language strings. This is the foundation of Moodle's localization and language support. All of the strings displayed to the end user are mapped via this facility. Language string mappings are also stored in the Moodle data lang folder. This structure allows for easy local customization of language files.

The following is a small section of the /lang/en_utf8/moodle.php language file. Notice how each string that is displayed to the end user is mapped to a string hash by using a key value that is descriptive of the strings purpose (in English):

$string['abouttobeinstalled'] = 'about to be installed';
$string['action'] = 'Action';
$string['actions'] = 'Actions';
$string['active'] = 'Active';
$string['activeusers'] = 'Active users';

lib:

The lib folder stores the core system library functions. As we develop modules and customizations, we will use classes and functions defined in this folder.

mod:

The mod folder stores activity modules such as assignment, quiz, wiki, forum, and lesson modules. Learning activities are the core of any course delivered using Moodle. Activity modules are more challenging to create than blocks, because they back up, restore, and store grades. Oh, and of course, they have to teach something to the learner.

my:

my is a light-weight portal area in Moodle. It provides a listing of courses a learner is assigned, including a summary of upcoming course activities. The user can also add and remove blocks on his or her portal page. my provides a good location to display custom information with minimal core changes to Moodle. For example, we use my as a dashboard location in many of our customization projects.

theme:

The theme folder stores all of the built-in Moodle themes and any custom themes installed on the system. Themes are a combination of CSS, HTML, and PHP. Each theme has its own folder. The theme system is useful for defining the visual skin, header, and footer of the Moodle page. It is, however, limited in how much of the Moodle page it can modify. For example, certain components of the Moodle page are hardcoded to display in a certain way.

The Moodle database is organized into roughly 200 related tables. The default installation option prepends mdl_ in front of each table name. Each major component of the system typically has one or more tables, each starting with the component's name. For example, there are two tables related to the config component: mdl_config and mdl_config_plugins. As programmers, we will have to manipulate the database on a regular basis. It's also important for us to be able to treat the entire database as an entity, copying and moving instances of an entire Moodle database for the creation of staging and testing areas as we develop our code. We generally do this by using command-line utilities such as mysqldump and MySQL.

Moodle data is the file storage location for user-uploaded content. Moodle data also stores the session data for users logged into the system, if file-based sessions are being used. Moodle data also stores optional language packs that can be downloaded from Moodle's administration interface. Moodle structures the data in this folder by either the user or by the course. Most of the data by file count and size will be in the courses. Each course has a folder, which is named with an integer value. The integer value is set to the internal database ID of the course in question. We can easily determine these values by navigating to a course via the Moodle web interface and inspecting its URL. For example, examine this URL for a course on my local test Moodle

http://localhost/workspace/moodle19/course/view.php?id=3.

Note the id=3 at the end of the URL. If we have uploaded any files to this course, there will be a folder path_to_moodledata/3. Within a course folder, Moodle will store module data in the moddata folder. When a module needs to store files, it saves them here in a folder with the same name as the module. For example, the wiki module will have a folder here named wiki. Additionally, Moodle will create a folder called backupdata if any course backups have been created. Any files that have been uploaded directly by a user using the course files interface will be loaded into the root of this folder. Users can also create their own folders and sub folders within the root folder.

Moodle 2.0 uses an entirely new organizational model for user-uploaded files, which is based on a hashing algorithm. The primary goal of this new method is to support efficient use of disk storage space and greater flexibility for using files across multiple courses.