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This article assumes that you've read Writing a Plugin, giving an overview (and many details) of how to develop a plugin. The same speaks specifically about the API "Hooks", also known as "Filters" and "Actions", used by WordPress to set your plugin at runtime.
These hooks can also be used in themes, as described here.
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 WordPress Hooks Database.
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| Actions Functions |
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| Activation/Deactivation/Uninstall Functions | |
|---|---|
Actions are triggered by specific events that take place in WordPress, such as publishing a post, changing themes, or displaying an administration screen. An Action is a custom PHP function defined in your plugin (or theme) and hooked, i.e. set to respond, to some of these events. Actions usually do one or more of the following:
The basic steps to make 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) {
$friends = '[email protected],[email protected]';
mail($friends, "sally's blog updated",
'I just put something on my blog: http://blog.example.com');
return $post_ID;
}
For most actions, your function should accept a single parameter (usually the post or comment ID, depending on the action). Some actions take more than one parameter -- check the documentation for the action (if available) or the WordPress source code for more information. Besides the one parameter, you can also access the global variables of WordPress, and call other WordPress functions (or functions in your plugin file).
Any text output by the function (e.g. with print) will appear in the page source at the location where the action was invoked.
NOTE: Keep in mind that other plugins or the WordPress core may already be using the function name you have thought of. See the next section, Avoiding Function Name Collisions for more information.
It is possible that someone has created a plugin with a function named the same as one in your plugin.
This is a problem because PHP does not allow multiple functions with the same name. If two plugins provide a function with the same name, or a plugin provides a function with a name the same as a WordPress function, the blog could cease to function. There are two ways to avoid this problem.
The first solution is to prefix every function in your plugin with a unique set of characters. If your name is John Q. Public, you might declare your functions as function jqp_output() {...}. The likelihood that someone with the same initials does the same thing with their plugin is possible but low.
The second - and possibly easier - a solution is to enclose your plugin functions in a class and call the class methods statically. This sounds more complicated than it is.
Consider this class, which expands on the examples provided above:
class emailer {
static function send($post_ID) {
$friends = '[email protected],[email protected]';
mail($friends,"sally's blog updated",'I just put something on my blog: http://blog.example.com');
return $post_ID;
}
}
add_action('publish_post', array('emailer', 'send'));
This class called emailer has a method send that implements the plugin functionality.
The add_action() function outside of the class adds the action to WordPress that tells it to call the send method when a post is published. The array used in the second parameter tells the plugin system to call the static method of the class 'emailer' named 'send'.
The function send is protected from the global namespace by the class declaration. It is not possible to call send() directly, and so any other function named send will not collide with this one. If you did want to call send(), you would need to use a scope resolution operator, like this: emailer::send()
The above example is for static methods. If you have an instance of a class then that won't work. To call a method of an instance you need to pass the instance as a variable. Consider the above example modified to take this into account:
class emailer {
function send($post_ID) {
$friends = '[email protected],[email protected]';
mail($friends,"sally's blog updated",'I just put something on my blog: http://blog.example.com');
return $post_ID;
}
}
$myEmailClass = new emailer();
add_action('publish_post', array($myEmailClass, 'send'));
Classes are a complicated subject. Read more about them in the PHP documentation on classes.
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 add 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, 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 (for example) needs to change a default option on activation, 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.
The register_uninstall_hook gives your plugin the option of cleaning up after itself when it is deleted from the WordPress installation. Users have reasons to temporarily deactivate plugins, so do not do anything from the deactivation hook that would lose any user settings. This is what the uninstall hook is for.