add_shortcode( 'MY_SHORTCODE', 'my_shortcode' );
function my_shortcode( $attr ) {
    $contents = '';
    ob_start();
    require_once('myfolder/show.php');
    $contents = ob_get_contents();
    ob_end_clean();
    return '[raw]'.$contents.'[/raw]';
    // Raw could be usefull to disable wordpress formatting (add <p>)
    // Voir http://www.catswhocode.com/blog/10-incredibly-cool-wordpress-shortcodes => POINT 9
}