add_filter

Quick Simple example of add_filter – sticking something in the bottom of the_content().

// filter to add stuff to the end of content in posts
function add_stuff_to_content($content) {
global $post;
	if ( 'post' != get_post_type() ) { // limit this to only the post type you want to alter
		return $content; //exit;
	}
	$original = $content;
	$morestuff .= '<div class="stuff">Extra Stuff!!</div>';
	
	$thecontent = $original.$morestuff; // puts more stuff right after post
	return $thecontent;
}
add_filter(the_content, add_stuff_to_content);