Using add_filter hook in WordPress — a working example

The add_filter hooks are being used in WordPress themes, in WordPress plugins and in WordPress core.

WordPress hooks change how the WordPress behaves by introducing functions without needing to edit any actual WordPress code.

(Apart from add_filter hooks there are also add_action hooks described here: add_action hooks in WordPress.

Here is the WordPress example for the add_filter hook.

Code example:

/*appending a list of child pages*/
function append_child_pages($content) {
if (is_page()) {
global $post;
$children = ''.wp_list_pages('title_li='.'&child_of='.$post->ID).'';
}
return  $children. $content;
}
add_filter('the_content','append_child_pages');

/*simple filter that adds author name to the contnent of a post.*/
function post_signature($content) {
global $post;
$author_id = $post->post_author;
$author = get_userdata($author_id);
$name = ''.$author->display_name.'';
return $content. 'by ' . $name;
}
add_filter('the_content','post_signature');

Now what are the other possible places different from the_content where we can set add filter hooks?

I will list them here:

• wp_title: allows the title tag to be altered or replaced
• the_title: allows the “title” of the post or page to be altered or replaced
• the_content: alters the content of the post or page
• wp_autop: automatically turns line breaks into paragraph tags
• do_shortcodes: processes short codes
• the_excerpt_length: determines the length (in characters) of the_excerpt()
• the_excerpt_more: determines what’s shown at the end of the_excerpt()
• wp_list_pages: allows the list of pages to be modified

Here is the list of all filter hooks:

It would be very good to mention here that it is possible in WordPress to get the list of all functions for the particular hook. In case of our the_content hook the result would be like this:

function get_filter_functions( $hook = '' ) {
    global $wp_filter;
    if( empty( $hook ) || !isset( $wp_filter[$hook] ) )
        return;
    else
    return $wp_filter[$hook];
}
print_r(get_filter_functions( 'the_content' ));

Summary: We updated the original page content with the additional list of subpages and with author name. You can alter your WordPress theme file further by adding new add_filter hooks. Finally, we outlined how to get filter functions for the specific hook.