content-rickroll.php

<?php
/*
Plugin Name: Rick Roll YouTube URLs
Description: Exactly what it says.
Author: Jan Dembowski
*/

add_filter( 'the_content' , 'mh_rick_roll_content' , 0 );
function mh_rick_roll_content( $content ) {
        /*
        Replace YouTube videos with a Rickroll
        http://www.youtube.com/watch?v=dQw4w9WgXcQ
        */

        $mh_url_regex = "/http\:\/\/www\.youtube\.com\/watch\?v\=[a-zA-Z0-9\-]+/";
        preg_match_all( $mh_url_regex , $content, $mh_matches );

        for ( $mh_count = 0; $mh_count < count( $mh_matches[0] ); $mh_count++ )
        {
                $mh_old_url = $mh_matches[0][$mh_count];
                $mh_new_url = 'http://www.youtube.com/watch?v=dQw4w9WgXcQ';
                $content = str_replace( $mh_old_url , $mh_new_url , $content );
        }

        return $content;
}