add to end of post in the loop with plugin

by

It sounds like you want a filter on the_content.

function add_content_wpse_97277($content) {
  global $post;
  if ($post->ID == 123) {
    $content .= 'additional content';
  }
  return $content;
}
add_filter('the_content','add_content_wpse_97277');

I don’t know what conditions you need though. You did not explain that in the question. The above should match a post with an ID of “123”.

For index pages you may need to add the same callback to the the_excerpt hook. You can control where the function adds content with template tags like is_home or is_single.

Honestly, this is how you should be adding your content both above and below the post. Echoing content from an action like the_post can work but is also a good way to get yourself in trouble. In this case I’d say it is particularly dangerous. the_post runs inside setup_postdata which does frequently run inside a loop that echoes content immediately, but if that function– setup_postdata— runs inside a Loop that doesn’t echo immediately, for example to build a string, your markup will make a mess. There is also loop_start and loop_end but I don’t think those are what you want.

Having seen your question and your comments and considered them, I have to say that your basic approach is wrong. You should be using the the_content and the the_excerpt, and/or possibly the get_the_excerpt filter as well.

Unfortunately filtering the content/excerpt doesn’t do what I need it
to do. There are often divs or footers added to a post (in the loop)
below the excerpt/content. That means filtering the content/excerpt
would not make my content appear at the bottom of the post in the
loop. Thanks though, your help is appreciated!

quoted from a comment below

I doubt there is a reliable way to do what you want. The only filter you have to work with that I can see is the_post. You could use that to insert between posts by tracking which post of the Loop you are on and inserting content conditionally. That is:

  1. If post number 0, then insert the begin <div>
  2. If post number 1, then insert </div> and <div>
  3. …
  4. If last post, then insert </div>

But again, you are going to make a mess echoing content from the the_post filter when setup_postdata is used in a context that does not echo content immediately– for example, in a context where a string is being built and returned.