19

In what order are plugins loaded in WordPress?

And within a particular plugin's folder, what order is followed for loading?

24

Answer to the First question:

  1. In wp-settings.php, WordPress first checks for any must-use plugins (plugins in the optional mu-plugins folder) and loads those.

  2. Then, if you're running a multisite installation, it checks for plugins that are network-activated and loads those.

  3. Then it checks for all other active plugins by looking at the active_plugins entry of the wp_options database table, and loops through those. The plugins will be listed alphabetically.

Here's the order WordPress loads pretty much everything: http://codex.wordpress.org/Action_Reference#Actions_Run_During_a_Typical_Request

The thing is, it usually doesn't matter what order each individual plugin is loaded in, because properly-written plugins will use WordPress hooks, which let you plug in functionality into specific points later in the WordPress startup. (Properly-written plugins will also prefix their functions and classes so that there aren't any conflicts.)

More info on plugin API: http://codex.wordpress.org/Plugin_API/

Answer to the Second question:

Totally depends on the plugin. WordPress only loads one file in the plugin, the one that's usually named the-plugin-name.php and contains the title, description, author, etc. at the top. It's up to the plugin to load the rest of its files, using require_once and wp_enqueue_script and whatnot.

  • So if I create a plugin, then later release an add-on plugins that should run after the original plugin is run, how can I ensure that order? – urok93 Aug 27 '12 at 6:21
  • It really depends what you're trying to do. But generally you would set up each of them to run on a specific hook using the add_action() function, which allows you to set priority. codex.wordpress.org/Function_Reference/add_action – SeventhSteel Aug 27 '12 at 13:31
  • @SeventhSteel But what if a plugin relies on the hook created by another plugin. If one plugin uses apply_filters() to create a hook, but runs before a plugin that uses add_filter(), then will it still work? Logically, it seems that apply_filters() will have fired already, so the add_filter() calls in the second plugin won't do anything. Does it matter in what order the plugins load when using only plugin-created hooks? – trusktr Jul 1 '13 at 17:15
  • What matters is what has loaded by the time apply_filters() runs. add_filter() must run before apply_filters(). If add_filter() is in a plugin file and not in another function, it will run on the plugins_loaded hook which is generally going to happen before you'd need apply_filters(). – SeventhSteel Nov 21 '14 at 17:28
  • A common scenario is having several plugins that append their content after the_content -- how is the order of their appearance determined? – Dan Knauss Mar 23 '17 at 16:12
9

I find that it is useful to include a 'late loading' action in my plugin that runs after all plugins have completed their load like this:

add_action('plugins_loaded', 'my_late_loader');

The function my_late_loader then is initiated after all other plugins which allows me to use hooks defined by other plugins in my own function my_other_function which is initiated within my_late_loader

/**
 * Late loading function for actions that must run after all plugins 
 * have loaded
 */
function my_late_loader(){
    add_action( 'some_hook', 'my_other_function', 10, 1);
}

Bit convoluted (I know), but this ensures that hooks in other plugins are created before being added irrespective of the plugin load order.

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