Guide to Finding Appropriate Hooks in a WordPress Theme or Plugin

To find the right hook in a WordPress theme or plugin, start from the exact output you want to change (like the featured image). Then follow the functions that generate that output until you find the first apply_filters() or do_action() that affects it. That hook is usually the cleanest place to customize behavior without editing core files. This guide shows the process step-by-step using a featured image example.

How often do you find yourself searching for an action or filter hook? It’s pretty much every time you have to make a change. May it be when creating an add-on plugin or a child theme. Wouldn’t you agree? For example, say you have to change the featured image of a post dynamically. How would you go about it?

What I’ll do is I’ll set a definite goal. Then we’ll go about and track down the exact hooks needed to resolve our issue.

Example Scenario: Display a Specific Featured Image for a Post

So, here’s what we’ll try doing. We’ll display a specific image — say attachment ID 4 — for post ID 27.
(Just an example — your IDs could differ if you’re testing this out.)

Extra detail: the current featured image for the post is attachment ID 12.

Quick Tip: How to find the id of an image/attachment?

• Go to the Media Library

• Select any image

• Check the URL. It will look like: http://example.com/wp-admin/post.php?post=4&action=edit

• The value associated with post= is the attachment ID.

Now here’s the constraint: you can’t make any changes in the database to change the featured image.

So, the only option you have (and you know it) is to find a suitable filter hook to replace the featured image dynamically.

The featured image of a post is displayed on the frontend using either the get_the_post_thumbnail() or the_post_thumbnail() functions. These functions locate the post’s featured image ID and then return the HTML needed to display the image.

The mapping between the featured image and the post is stored in wp_postmeta. The key _thumbnail_id is used to store the mapping, and the value associated with it is the attachment ID.

So, in our case, for post 27, _thumbnail_id would currently resolve to 12.

All good so far? Moving on.

Seek and ye shall find! How to Track Down the *Perfect* Hook

We now know WordPress will display the featured image by reading the value set for _thumbnail_id.
So the goal is simple:

✅ intercept _thumbnail_id before WordPress uses it to render the image.

The best place to start is inside the functions responsible for outputting featured images:

• get_the_post_thumbnail() (returns HTML)

• the_post_thumbnail() (echoes that HTML — it’s basically a wrapper)

For the sake of this tutorial, we’ll trace the get_the_post_thumbnail() function until we find the first useful filter.

Why this method works Most WordPress features are built as a chain of functions. Hooks are intentionally placed inside that chain so developers can change behavior safely, without editing WordPress core, the theme, or the plugin directly.

Step 1: Check for a Filter before a Value is returned

Open the post-thumbnail-template.php file and find the function definition for get_the_post_thumbnail. You will notice, this function calls the get_post_thumbnail_id function to get the thumbnail id.

get_post_thumbnail_id( $post_id );

The output of this function is assigned to variable $post_thumbnail_id.

If there was filter before the return value, then writing code on that filter would work for us. But alas! there is no such filter. So what’s next? We have to take a look at the nested function inside the current function. In our case this function would be the get_post_thumbnail_id function.

Quick takeaway: If there’s no hook where the value is returned, you’ll need to check the next function it calls.

Step 2: Check for a Filter in the Nested function

Locate the get_post_thumbnail_id function. It is defined in the same file. Check the return value of the function. The function returns the result of get_post_meta function.

return get_post_meta( $post_id, '_thumbnail_id', true );

But you’re faced with the same problem here. There isn’t a filter before the return value, and hence it can’t be changed.

“PATIENCE YOU MUST HAVE my young padawan”

I’m sure you know what we have to do next.

We need to dig into the get_post_meta function. This function searches for the value of _thumbnail_id in the database for $post_id. We need to get a filter here.

Lets get into that function right away.

Quick takeaway: No hook here either? That’s normal. Keep following the chain—WordPress often places hooks deeper in shared functions.

Step 3: Keep digging deeper!

The get_post_meta function is defined in wp-includes/post.php and it contains only one line:

return get_metadata('post', $post_id, $key, $single);

But no filter there which means we will have to dig into the function get_metadata.

The get_metadata function is defined in wp-includes/meta.php. This function has has just what we need. Awesome!

$check = apply_filters( "get_{$meta_type}_metadata", null, $object_id, $meta_key, $single );

Writing our code on this filter will do the job. We’ll return the image id we need (12). Let’s go for it.

In our case {$meta_type} is post since we are dealing with post data. Therefore, our code should be as follows:

add_filter( 'get_post_metadata', 'wdm_change_featured_image', 10, 4 );
/*
* Sets featured image dynamically
*
* @param int $featured_image_id  Image/Attachment ID. Default value is null
* @param int $post_id  Post id of the post
* @param string $meta_key  Meta key for which value to be set
* @param string $single Meta value, or an array of values.
* @return mixed Returns the featured image id
*/
function wdm_change_featured_image( $feature_image_id = null, $post_id, $meta_key, $single ) {
    //we have to return 12 if post id is 27
    if ( $post_id == 27 && meta_key == '_thumbnail_id' ) {
      return 12;
    }
    return $feature_image_id;
}

This code can be added in functions.php of your theme or in a custom plugin. Either way you’re set.

If you’re lucky, the filter you’re looking for will be in the first place you look. If not, I hope the way I tracked the filter will help you overcome a few hurdles. And be sure to share helpful tips for fellow developers in the comment section below!

Quick takeaway: Once you find the first relevant apply_filters()/do_action(), you’ve found a safe entry point for customization.

Conclusion

If you’re lucky, the hook you need will be sitting right where you first look.

If not, the method is always the same: start at the output, trace the function calls, and keep digging until you hit an apply_filters() or do_action() that gives you a clean entry point.

Once you get used to doing this a few times, finding hooks stops feeling like guessing and starts feeling like a repeatable process.

Need help implementing this safely?
If you’re adding hook-based customizations on a business-critical site, it’s worth doing it in a way that stays update-safe and easy to maintain. If you’d like, our team can help you implement it cleanly without breaking updates.

Get in touch with us today!

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