这是用于gallery短代码的wordpress函数，我想了解它是如何工作的，这样我就可以实现一个幻灯片短代码，并像gallery短代码那样添加img id。

    function gallery_shortcode($attr) {
$post = get_post();

static $instance = 0;
$instance++;

if ( ! empty( $attr[\'ids\'] ) ) {
    // \'ids\' is explicitly ordered, unless you specify otherwise.
    if ( empty( $attr[\'orderby\'] ) )
        $attr[\'orderby\'] = \'post__in\';
    $attr[\'include\'] = $attr[\'ids\'];
}

// Allow plugins/themes to override the default gallery template.
$output = apply_filters(\'post_gallery\', \'\', $attr);
if ( $output != \'\' )
    return $output;

// We\'re trusting author input, so let\'s at least make sure it looks like a valid orderby statement
if ( isset( $attr[\'orderby\'] ) ) {
    $attr[\'orderby\'] = sanitize_sql_orderby( $attr[\'orderby\'] );
    if ( !$attr[\'orderby\'] )
        unset( $attr[\'orderby\'] );
}

extract(shortcode_atts(array(
    \'order\'      => \'ASC\',
    \'orderby\'    => \'menu_order ID\',
    \'id\'         => $post ? $post->ID : 0,
    \'itemtag\'    => \'dl\',
    \'icontag\'    => \'dt\',
    \'captiontag\' => \'dd\',
    \'columns\'    => 3,
    \'size\'       => \'thumbnail\',
    \'include\'    => \'\',
    \'exclude\'    => \'\',
    \'link\'       => \'\'
), $attr, \'gallery\'));

$id = intval($id);
if ( \'RAND\' == $order )
    $orderby = \'none\';

if ( !empty($include) ) {
    $_attachments = get_posts( array(\'include\' => $include, \'post_status\' => \'inherit\', \'post_type\' => \'attachment\', \'post_mime_type\' => \'image\', \'order\' => $order, \'orderby\' => $orderby) );

    $attachments = array();
    foreach ( $_attachments as $key => $val ) {
        $attachments[$val->ID] = $_attachments[$key];
    }
} elseif ( !empty($exclude) ) {
    $attachments = get_children( array(\'post_parent\' => $id, \'exclude\' => $exclude, \'post_status\' => \'inherit\', \'post_type\' => \'attachment\', \'post_mime_type\' => \'image\', \'order\' => $order, \'orderby\' => $orderby) );
} else {
    $attachments = get_children( array(\'post_parent\' => $id, \'post_status\' => \'inherit\', \'post_type\' => \'attachment\', \'post_mime_type\' => \'image\', \'order\' => $order, \'orderby\' => $orderby) );
}

if ( empty($attachments) )
    return \'\';

if ( is_feed() ) {
    $output = "\\n";
    foreach ( $attachments as $att_id => $attachment )
        $output .= wp_get_attachment_link($att_id, $size, true) . "\\n";
    return $output;
}

$itemtag = tag_escape($itemtag);
$captiontag = tag_escape($captiontag);
$icontag = tag_escape($icontag);
$valid_tags = wp_kses_allowed_html( \'post\' );
if ( ! isset( $valid_tags[ $itemtag ] ) )
    $itemtag = \'dl\';
if ( ! isset( $valid_tags[ $captiontag ] ) )
    $captiontag = \'dd\';
if ( ! isset( $valid_tags[ $icontag ] ) )
    $icontag = \'dt\';

$columns = intval($columns);
$itemwidth = $columns > 0 ? floor(100/$columns) : 100;
$float = is_rtl() ? \'right\' : \'left\';

$selector = "gallery-{$instance}";

$gallery_style = $gallery_div = \'\';
if ( apply_filters( \'use_default_gallery_style\', true ) )
    $gallery_style = "
    <style type=\'text/css\'>
        #{$selector} {
            margin: auto;
        }
        #{$selector} .gallery-item {
            float: {$float};
            margin-top: 10px;
            text-align: center;
            width: {$itemwidth}%;
        }
        #{$selector} img {
            border: 2px solid #cfcfcf;
        }
        #{$selector} .gallery-caption {
            margin-left: 0;
        }
        /* see gallery_shortcode() in wp-includes/media.php */
    </style>";
$size_class = sanitize_html_class( $size );
$gallery_div = "<div id=\'$selector\' class=\'gallery galleryid-{$id} gallery-columns-{$columns} gallery-size-{$size_class}\'>";
$output = apply_filters( \'gallery_style\', $gallery_style . "\\n\\t\\t" . $gallery_div );

$i = 0;
foreach ( $attachments as $id => $attachment ) {
    if ( ! empty( $link ) && \'file\' === $link )
        $image_output = wp_get_attachment_link( $id, $size, false, false );
    elseif ( ! empty( $link ) && \'none\' === $link )
        $image_output = wp_get_attachment_image( $id, $size, false );
    else
        $image_output = wp_get_attachment_link( $id, $size, true, false );

    $image_meta  = wp_get_attachment_metadata( $id );

    $orientation = \'\';
    if ( isset( $image_meta[\'height\'], $image_meta[\'width\'] ) )
        $orientation = ( $image_meta[\'height\'] > $image_meta[\'width\'] ) ? \'portrait\' : \'landscape\';

    $output .= "<{$itemtag} class=\'gallery-item\'>";
    $output .= "
        <{$icontag} class=\'gallery-icon {$orientation}\'>
            $image_output
        </{$icontag}>";
    if ( $captiontag && trim($attachment->post_excerpt) ) {
        $output .= "
            <{$captiontag} class=\'wp-caption-text gallery-caption\'>
            " . wptexturize($attachment->post_excerpt) . "
            </{$captiontag}>";
    }
    $output .= "</{$itemtag}>";
    if ( $columns > 0 && ++$i % $columns == 0 )
        $output .= \'<br style="clear: both" />\';
}

$output .= "
        <br style=\'clear: both;\' />
    </div>\\n";

return $output;