Inside a well-built Moodle plugin
Pick the type before writing a line
Moodle has dozens of plugin types and each one plugs into a different part of the system. An activity module (mod) lives inside courses, has grades and completion, and must support backup. A block is a panel that can sit on the dashboard or a course page. An enrol plugin controls how users get into courses, and an auth plugin controls how they log in. Reports, course formats and question types each have their own base classes. A local plugin is the catch-all for site-wide rules and integrations. Putting a feature into the wrong type is the most common design mistake we see, and it is expensive to move later.
Files Moodle expects
A plugin announces itself in version.php with its component name, its version and the Moodle release it requires. Language strings sit in the lang folder, never hard-coded. The db folder holds install.xml for tables, upgrade.php for schema changes, access.php for capabilities, events.php for observers, tasks.php for scheduled tasks and services.php for web service functions. Classes are namespaced and autoloaded. Output goes through Mustache templates and renderers so that a theme can restyle it. We follow that layout exactly, because it is what lets administrators, other developers and Moodle itself understand the plugin.
Security is the capability check you did not skip
Every page starts by requiring login and checking a capability in a context. Every form carries a session key. Input arrives through typed parameters, and queries use placeholders through the DB API instead of concatenated SQL. Output is escaped by the template. Moodle treats all of this as mandatory, and it is the reason a student cannot open the teacher view by changing a URL.
The parts that get skipped
Three pieces are often missing from plugins we are asked to fix. The privacy API, which tells Moodle what personal data the plugin stores and how to export or delete it for a data request. Backup and restore, without which a course copy silently drops the plugin's data. And upgrade steps with savepoints, so that a schema change applies once and in order on every site. We scope all three from the start. When the plugin has to be reachable from other systems or from the app, we also expose external functions, as described on our Moodle API development page.
Testing and style
Code is checked with Moodle's code sniffer rules and PHPDoc checks on every commit, alongside PHPUnit tests that use Moodle's data generators and Behat scenarios that click through the feature as a teacher and as a student. The same pipeline runs against each Moodle release and database you want supported. That is how we can tell you, before an upgrade, whether your plugin is ready.