In what order does WordPress load plugin files?

4 comments

1. 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.

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2. 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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3. I originally came to this page with the same question as the first one. Both the answers by @SeventhSteel and @Clinton provide some very useful data, but require additional digging. As stated in those answers, your properly written plug-in will use hooks, and will have a call something like the following):

   add_action('init', 'function_to_add');
   

   The first parameter is “the name of the action” to which your function — the second parameter — is hooked. The action specified is the main determinant of when your plugin is loaded. There is an optional third parameter (priority) which gives you some granular control over where within that group your function is loaded. (Though this is hardly precise since it would often be difficult to know the priorities of all installed plugins.) There is an excellent list of all the available action hooks with explanations thereof for various request types in the WordPress Codex.

   As for the second question, as stated in the Plugin Basics documentation:

   When WordPress loads the list of installed plugins on the Plugins page of the WordPress Admin, it searches through the plugins folder (and its sub-folders) to find PHP files with WordPress plugin header comments.

   So the file in your plug-in with the header (per the documentation, there should be only one) will be loaded first. See @Clinton’s answer for suggestions on handling multiple hooks within a single plug-in.

   Should you come to this answer (as I did) as a first-time WordPress plugin developer, there’s a lot more useful information to be found in WordPress’ Plugin Handbook.

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4. About dependency between plugins, I found the easiest way to control to order they load is by wrapping the init with:

   add_action('init', 'init_first', 11); in first_plugin.php
   
   add_action('init', 'init_second', 12); in second_plugin.php
   

   So the actual order they load might be still random, but the order they run will be determined by the priority you give.

   Yaakov.

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