Inserting code after each post in WordPress

Question

I am wanting to insert some code after each post in WordPress... I know you can do it after this for example, in single.php

Answer 1

Here is an example of a filter on the content and the function:

    function the_content_replacer($content) 
    {
//global $post, $posts;
       //$content = str_replace(']]>', ']]>', $content);

 $content .= "\n<div style=\"display:none;\">text here</div>"; 

//$content = preg_replace('/="http:\/\/cnn/i', 
// '="http://example.com?http://cnn', $content, -1); 
       return $content;
    }
    add_filter('the_content', 'the_content_replacer', 1);

1. much more examples on this filter on http://wordpress.stackexchange.com ..........
2. You can just copy and paste the piece of content in the file "functions.php" in your theme and it will work.
3. You can also just drop it in the directory wp-content/mu-plugins if your run multisite so it works on all the blogs in your multisite environment.
4. the third parameters determines the importance of when applying the filter, see: https://wordpress.stackexchange.com/questions/2126/at-what-priority-does-add-filter-overwrite-core-functions

--> it is better to post all WordPress questions in http://wordpress.stackexchange.com !!!

--> if you use e.g.

$content .= "\n<div style=\"display:none;\">text here</div>";

it will not remove a closing paragraph tag (note the linebreak at the beginning of the string)