in WordPress, add_filter() is a function used to hook a custom function or an existing WordPress function to a specific filter action. Filters are one of the two types of Hooks, the other being Actions. They provide a way for functions to modify the data of other functions and are the cornerstone of WordPress’ plugin functionality.
Here’s the basic syntax of add_filter():
add_filter( string $tag, callable $function_to_add, int $priority = 10, int $accepted_args = 1 )As a content management system (CMS), WordPress was designed as any other platform… wether it’s a page, a post, or even a custom post you have a title and content. But not all content is limited to those two options. One example is the acronym library that I’ve developed on Martech Zone. An acronym has three elements… the acronym itself, the definition that shows what the acronym stands for, and an explanation of it.
I was able to easily add a custom field for the definition using MetaBox, but that custom field isn’t published throughout the site. One means of doing it would be to build a custom template for the archive and single acronym page where I can extract the custom field. However, that requires quite a bit of work and has to be done wherever I want that information – in the archive, the single post, the excerpt, and the feed of the custom post type.
An alternative is to use your theme and prepend that information within the content itself. In this case, I simply want to prepend a short sentence: The {title} is the acronym for {definition}. Because I also use the acronym library for codes, I also want to modify the prepended text if the code is numeric: The {title} is the code for {definition}. Here’s are examples:
0p is the acronym for Zero Party and 404 is the code for Not Found.
To do this, I can utilize add_filter for the concept, excerpt, feed, and RSS to prepend the appropriate text. Additionally, I check to see if the acronym is numeric… in which case it’s likely a code. (I realize I could enhance this further, but for now it’s fine). Within the functions.php file of my child theme, I simply add the following function and then call the appropriate filters to apply it throughout the site:
// Prepend text to the content of 'acronym' posts
add_filter('the_content', 'prepend_text_to_acronym');
add_filter('the_excerpt', 'prepend_text_to_acronym');
add_filter('the_content_feed', 'prepend_text_to_acronym');
add_filter('the_excerpt_rss', 'prepend_text_to_acronym');
function prepend_text_to_acronym($content) {
global $post;
// Check if it's an 'acronym' post
if($post->post_type == 'acronym') {
// Get the post title and the 'acronym_definition' field
$title = get_the_title($post->ID);
$definition = get_post_meta($post->ID, 'acronym_definition', true);
if (is_numeric($title)) {
$new_content = "$title is the code for $definition.
";
} else {
$new_content = "$title is the acronym for $definition.
";
}
// Prepend the new content to the original content
$content = $new_content . $content;
}
return $content;
}
Now, when you see my Acronym archive, you’ll see each of the entries have that sentence prepended in the excerpt. And it’s a standalone paragraph on the single posts page.
