Testing WP hooks

test-wp-hooks.php

<?php 
/**
 * Plugin Name: Testing
 */
// Exit if accessed directly.
if ( ! defined( 'ABSPATH' ) ) {
    exit;
}

// Hook into wp_footer to trigger the custom action
add_action('wp_footer', 'testing_callback');

function testing_callback() {
    do_action('footer_message');
}

// Filter the content with the custom filter

add_filter('the_content', 'testing_filter_callback');
function testing_filter_callback($content) {
    $content = apply_filters('wp_hooks_guide_filter', $content);
    return $content;
}