You’ll want to hook into the_content
to add a message.
Something like this:
<?php
add_filter('the_content', 'wpse51338_filter_content');
/*
* Filter the content to add a message before and after it.
*/
function wpse51338_filter_content($content)
{
// not a singular post? just return the content
if(!is_singular())
return $content;
$msg = sprintf(
'<p class="special-message">%s</p>',
__('Here is an awesome message!', 'your_textdomain_string') // this is your message
);
return $msg . $content . $msg;
}
As a plugin.