We learned that filter hooks (also referred to as simply 'filters') are functions that process WordPress content, whether it is about to be saved in the database or displayed in the user's browser. WordPress expects these functions to modify the content they get and return it.

In our case, we used the_content filter hook to modify the post content by appending a Digg link to it. We could also have placed the Digg link at the beginning of the post, or broken up the post and put it in the middle.

To set up a filter, we need to use the add_filter function:

add_filter ( 'filter_hook', 'filter_function_name' , [priority], [accepted_args] );

Here is an example list of filter hooks, which will help you to get a better understanding of what you can achieve using them.

Filter hooks can be removed using the remove_filter() function. It accepts the same arguments as add_filter(), and is useful if you want to replace some of the existing WordPress filters with your functions.

If you want to take a closer look at the default WordPress filters, you can find them in the wp-includes\default-filters.php file of your WordPress installation.

We use action hooks when we need to include specific functionalities every time a WordPress event triggers, for example when the user publishes a post or changes the theme.

WordPress does not ask for any information back from the action function, it simply notifies it that a certain event has happened, and that a function should respond to it in a desired way.

Action hooks are used in a way similar to the filter hooks. The syntax for setting up an action hooks is:

add_action ( 'action_hook', 'action_function_name', [priority], [accepted_args] );

The following table presents example action hooks provided by WordPress.

Just as with filters, you can use the remove_action() function to remove currently registered actions.

Since understanding the power of filters and actions is very important for conquering WordPress plugin development, we will now examine a few more simple examples of their usage.

add_filter('the_title', strtoupper);

function mailing_list($post_ID)  
{
    $list = 'john@somesite.com,becky@somesite.com';    
    mail($list, 'My Blog Update', 
         'My blog has just been updated: '.get_settings('home'));    
}

// Send notification with every new post and comment
add_action('publish_post', 'mailing_list');
add_action('comment_post', 'mailing_list');

/* Create excerpt with 70 words and preserved HTML tags */
function my_wp_trim_excerpt($text) 
{ 
    if ( '' == $text ) 
    {
        $text = get_the_content('');
        $text = apply_filters('the_content', $text);
        $text = str_replace(']]>', ']]>', $text);        
        $excerpt_length = 70;
        $words = explode(' ', $text, $excerpt_length + 1);
        if (count($words) > $excerpt_length) 
        {
            array_pop($words);
            array_push($words, '[...]');
            $text = implode(' ', $words);
        }
    }
    return $text;
}

// remove WordPress default excerpt filter
remove_filter('get_the_excerpt', 'wp_trim_excerpt');

// Add our custom filter with low priority
add_filter('get_the_excerpt', my_wp_trim_excerpt, 20);