WordPress Hooks

Hooking-the-post-content.php

<?php

/**
* See the next link to understand more about this filter hook.
*
* @see https://developer.wordpress.org/reference/hooks/the_content/
* @param string $content - the post content.
* @return string $content - the content updated
*/
function add_footer_to_post_content( $content ) {
  $content = sprintf(
    '%s <p>My custom footer</p>',
    $content
  );
  
  return $content;
}

add_filter( 'the_content', 'add_footer_to_post_content', 10, 1 );