How can I check if a custom field was updated?

Question

I have a custom post named vpn. That custom post has a custom field named company_location.

Inside my functions.php file I am using the updated_post_meta hook to call a bg_update function I created.

I was expecting the function to var_dump the words it works at the top of my post edit screen after I change the company_location custom field on the post and click "update".

But nothing happens. No var_dump shows at all.

function bg_update ($meta_id, $object_id, $meta_key, $_meta_value) {
    $test = "it works";
    if ($meta_key == "company_location") {
        var_dump($test);
    }
}
add_action( 'updated_post_meta', 'bg_update', 10, 4 );

What am I doing wrong?

UPDATE:

This question here says that the post edit page redirects after loading, so you need to kill wp to see the var_dump. After playing with that I have found that this will kill wp and show the var_dump:

function bg_update ($meta_id, $object_id, $meta_key, $meta_value) {
    $test = "it works";
    var_dump($test);
    wp_die();
    if ($meta_key == "company_location") {
        // do stuff here
    }
}
add_action( 'updated_post_meta', 'bg_update', 10, 4 );

But this does nothing. WP is not killed and the var_dump is not showing:

function bg_update ($meta_id, $object_id, $meta_key, $meta_value) {
    $test = "it works";
    if ($meta_key == "company_location") {
        var_dump($test);
        wp_die();
    }
}
add_action( 'updated_post_meta', 'bg_update', 10, 4 );

So I now know that updated_post_meta is the correct hook because it is firing the function, but the problem is the IF statement is evaluating to false and therefore not running var_dump.

Why is that?

UPDATE 2:

I var_dumped $meta_key outside the IF statement and it's showing string(10) "_edit_lock"??

function bg_update ($meta_id, $object_id, $meta_key, $meta_value) {
    $test = "it works";
    var_dump($meta_key);
    wp_die();    
    if ($meta_key == "company_location") {
        // do things
    }
}
add_action( 'updated_post_meta', 'bg_update', 10, 4 );

Answer 1

I think you forgot the priority of hook and number of arguments otherwise code is perfect. If you can update post meta for custom post type then you refer this.

https://codex.wordpress.org/Plugin_API/Action_Reference/updated_(meta_type)_meta

Try Like this:

add_action( 'updated_post_meta', 'bg_update', 10, 4 );

For custom post type EX: abc

<?php add_action( "updated_{$meta_type}_meta", $function_name', 10, 4 ); ?>

<?php add_action( "updated_abc_meta", $function_name', 10, 4 ); ?>

UPDATE

function save_custom_field( $post_id, $post, $update ) {
//YOUR CODE
}
add_action( 'save_post', 'save_custom_field', 10, 3 );