Convert iTunes links to TradeDoubler affiliate links

/**
 * Convert iTunes links to TradeDoubler affiliate links
 */
add_filter( 'the_content', 'themename_music_link_filter', 1 );
add_filter( 'comment_text', 'themename_music_link_filter' );

function themename_music_link_filter( $content ) {
    $content = preg_replace_callback( '/<a\b(?>\s+(?:href="([^"]*)")|[^\s>]+|\s+)*>/', 'themename_affiliatize_links', $content );

    return $content;
}

function themename_affiliatize_links( $matches ) {
    $link_tag = $matches[0];

    if  ( isset( $matches[1] ) ) {
        if ( 0 === strpos( $matches[1], 'http://itunes.apple.com/' ) || 0 === strpos( $matches[1], 'http://phobos.apple.com/' ) ) {
            $wrapper = 'http://clkuk.tradedoubler.com/click?p=23708&a=1995293&url=';
            $new_url = add_query_arg( 'partnerId', 2003, $matches[1 ]);
            $new_url = $wrapper . rawurlencode( rawurlencode( $new_url ) );
            $link_tag = str_replace( $matches[1], $new_url, $matches[0] );
        }
    }

    return $link_tag;
}