Understanding how WordPress plugins work will be beneficial if you are interested in developing your own or extending existing ones to suit your needs.

What is a WordPress plugin?

A WordPress plugin alters or extends core functionalities that are built-in to WordPress. In other words, plugins are files that are installed, activated or sometimes created in a WordPress site to add a feature, or set of features and it can range from a simple calendar widget to a full-featured e-commerce system.

When are WordPress plugins loaded?

WordPress plugins are loaded early when you visit a page in a WordPress site, WordPress will go through the following standard loading process:

• First it will load the wp-config.php file located in the root directory.
• Second, it will load the functions.php file under the wp_includes directory.
• Third, it will load all the plugins that are installed and activated.
• Fourth, it will load the pluggable functions. These functions override specific core functions in a plugin. An example would be the wp_mail function. You can use this function in your plugin to send email using SMTP rather than the default mail PHP function. All pluggable functions are defined in the pluggable.php core WordPress file under the wp_includes directory.
• Fifth, it will load the translations that are done through internationalization and localization.
• Sixth, it will load the theme that is currently active.
• Lastly, it will load the page content.

WordPress Plugin Directories

WordPress features two plugin directories. The primary directory for plugins is located under wp-content/plugins. The second, lesser known plugin directory is located under wp-content/mu-plugins. MU which stands for “Must-Use” is not created by WordPress, it must be manually created to be used.

The plugins under the mu-plugins directory doesn’t have an activate/deactivate feature and will always be executed globally. The only way to deactivate one is by deleting the file completely with conviction…

WordPress Plugin Types and Statuses

WordPress features a few types and statuses for plugins:

• Active – A status that indicates that a plugin is active and running in WordPress.
• Inactive – A status that indicates that a plugin is installed but not active and no code from the plugin are being executed.
• Must Use – These are plugins that are created manually in the wp-content/mu-plugins directory and are loaded site-wide automatically.
• Drop-ins – These plugins replaces core WordPress functionalities, they are a specifically named PHP files located in the wp-content directory. Here’s some drop-ins that are available: db.php, db-error.php, install.php, maintenance.php, advanced-cache.php and object-cache.php.

Where to download WordPress plugins?

WordPress plugins can be downloaded (either paid or free) from a variety of sources like CodeCanyon and the Official WordPress Plugin Directory. All the plugins available in the plugin directory are 100% GPL and free to use for personal or commercial purposes.

Next Post: Action Hooks in WordPress

This is part 1 of the WordPress Plugin Development series.

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