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This page documents the API (Application Programming Interface) hooks available to WordPress plugin developers, and how to use them.
This article assumes you have already read Writing a Plugin, which gives an overview (and many details) of how to develop a plugin. This article is specifically about the API of "Hooks", also known as "Filters" and "Actions", that WordPress uses to set your plugin in motion. These hooks may also be used in themes, as described here.
Note: This information applies to WordPress Versions 1.2 and higher. Before Version 1.2, modifications were called "hacks" and involved editing the source code of WordPress itself.
Hooks are provided by WordPress to allow your plugin to 'hook into' the rest of WordPress; that is, to call functions in your plugin at specific times, and thereby set your plugin in motion. There are two kinds of hooks:
You can sometimes accomplish the same goal with either an action or a filter. For example, if you want your plugin to change the text of a post, you might add an action function to publish_post (so the post is modified as it is saved to the database), or a filter function to the_content (so the post is modified as it is displayed in the browser screen).
For a thorough listing of all action and filter hooks in WP see Adam Brown's WP Hooks Database.
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Actions are triggered by specific events that take place in WordPress, such as publishing a post, changing themes, or displaying a page of the admin panel. Your plugin can respond to the event by executing a PHP function, which might do one or more of the following:
The basic steps to making this happen (described in more detail below) are:
The first step in creating an action in your plugin is to create a PHP function with the action functionality of your plugin, and put it in your plugin file (your plugin file must go into the wp-content/plugins directory). For example, if you want your friends to get an email message whenever you create a new post, you might define the following function:
function email_friends($post_ID) {
$friends = 'bob@example.org,susie@example.org';
mail($friends, "sally's blog updated",
'I just put something on my blog: http://blog.example.com');
return $post_ID;
}
For most actions, your function should accept a single parameter (usually the post or comment ID, depending on the action). Some actions take more than one parameter -- check the documentation for the action (if available) or the WordPress source code for more information. Besides the one parameter, you can also access the global variables of WordPress, and call other WordPress functions (or functions in your plugin file).
Any text output by the function (e.g. with print) will appear in the page source at the location where the action was invoked.
NOTE: Keep in mind that other plugins or the WordPress core may already be using the function name you have thought of. See Avoiding Function Name Collisions for more information.
After your function is defined, the next step is to "hook" or register it with WordPress. To do this, call add_action() in the global execution space of your plugin file:
add_action ( 'hook_name', 'your_function_name', [priority], [accepted_args] );
where:
In the example above, we would put the following line in the plugin file:
add_action ( 'publish_post', 'email_friends' );
Likewise, you can also Remove Actions from action hooks. See that section for details.
The last step in getting your action hook to work is to install the file and activate the plugin. The PHP function you wrote and the add_action call must go into a PHP file together, and the PHP file must be installed in the wp-content/plugins directory. Once it is installed, you will need to visit the admin section of WordPress and activate your plugin; see Managing Plugins for more details.
See Plugin API/Action Reference for a current list of action hooks in WordPress, and links to previous versions of WordPress.
Filters are functions that WordPress passes data through, at certain points in execution, just before taking some action with the data (such as adding it to the database or sending it to the browser screen). Filters sit between the database and the browser (when WordPress is generating pages), and between the browser and the database (when WordPress is adding new posts and comments to the database); most input and output in WordPress passes through at least one filter. WordPress does some filtering by default, and your plugin can add its own filtering.
The basic steps to adding your own filters to WordPress (described in more detail below) are:
A filter function takes as input the unmodified data, and returns modified data (or in some cases, a null value to indicate the data should be deleted or disregarded). If the data is not modified by your filter, then the original data must be returned so that subsequent plugins can continue to modify the value if necessary.
So, the first step in creating a filter in your plugin is to create a PHP function to do the filtering, and put it in your plugin file (your plugin file must go into the wp-content/plugins directory). For example, if you want to make sure that your posts and comments contain no profanity, you might define a variable with a list of forbidden words, and then create the following PHP function:
function filter_profanity($content) {
$profanities = array('badword','alsobad','...');
$content=str_ireplace($profanities,'{censored}',$content);
return $content;
}
Why does this work without a loop? Because $profanities is an array, and str_ireplace loops through the array for you. The str_ireplace function is used instead of str_replace because str_ireplace is case insensitive.
NOTE: Keep in mind that other plugins or the WordPress core may already be using the function name you have thought of. See the Plugin Development Suggestions for more information.
After your function is defined, the next step is to "hook" or register it with WordPress. To do this, call add_filter() in the global execution space of your plugin file:
add_filter ( 'hook_name', 'your_filter', [priority], [accepted_args] );
where:
In the example above, we would put the following in the main executing section of the plugin file, to tell WordPress to filter comments for profanity:
add_filter('comment_text','filter_profanity');
You can also remove filters from filter hooks using the remove_filter() function. See Removing Actions and Filters.
The last step in getting your filter hook to work is to install the file and activate the plugin. The PHP function you wrote and the add_filter() call must go into a PHP file together, and the PHP file must be installed in the wp-content/plugins directory. Once it is installed, you will need to visit the admin section of WordPress and activate your plugin; see Managing Plugins for more details.
See Plugin API/Filter Reference for a current list of filter hooks in WordPress, and links to previous versions of WordPress.
This is an example, as described by Ozh on the wp-hackers email list, for a plugin to modify (or overwrite) the default bloginfo() function. This will require modifying a core function behavior.
add_filter('bloginfo', 'mybloginfo', 1, 2);
add_filter('bloginfo_url', 'mybloginfo', 1, 2);
function mybloginfo($result='', $show='') {
switch ($show) {
case 'wpurl':
$result = SITE_URL;
break;
case 'template_directory':
$result = TEMPL_DIR;
break;
default:
}
return $result;
}
In some cases, you may find that you want your plugin to disable one of the actions or filters built into WordPress, or added by another plugin. You can do that by calling remove_filter('filter_hook','filter_function') or remove_action('action_hook','action_function').
For example, remove_action('publish_post','generic_ping'); would prevent your weblog from sending pings whenever a new post is created.
Note that if a hook was registered using a priority other than the default of 10, then you must also specify the priority in the call to remove_action(). Also note that in general, you shouldn't remove anything unless you know what it does and why it does it -- check the WordPress or other plugin source code to be sure.
Besides the hooks (actions and filters) described above, another way for a plugin to modify WordPress's behavior is to override WordPress functions. In fact, there is a small set of functions WordPress intends for plugins to redefine. These are called Pluggable Functions and they are defined in wp-includes/pluggable.php. WordPress loads these functions only if they are still undefined after all plugins have been loaded. For more details examine wp-settings.php file.
If your plugin has tasks to complete only at activation or deactivation time, it can use register_activation_hook and register_deactivation_hook. Many plugins do not need to use these, as the plugins only modify current behavior. However, if your plugin needs to change a default option on activation or if it needs to upgrade from changes made by a previous version, it can use these functions.
Creating Tables with Plugins has an example using the register_activation_hook function to make the database compatible with the current version of the plugin.