At the top of the Gallery Shortcode Callback is a filter hook: post_gallery
If any non-empty / non-null output is returned from this filter the shortcode will use that output instead of the default generation. You can copy the source of the Gallery Shortcode Callabck and modify it as you see fit.
The below is a filter hook, using the entire code of the gallery callback with the post_gallery filter hook removed to prevent any infinite looping.
/**
* Custom Gallery Function for custom output
*
* @param string $output The gallery output. Default empty.
* @param array $attr Attributes of the gallery shortcode.
* @param int $instance Unique numeric ID of this gallery shortcode instance.
*
* @return String $output
*/
function my_custom_gallery( $output, $attr, $instance ) {
$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'];
}
$html5 = current_theme_supports( 'html5', 'gallery' );
$atts = shortcode_atts(
array(
'order' => 'ASC',
'orderby' => 'menu_order ID',
'id' => $post ? $post->ID : 0,
'itemtag' => $html5 ? 'figure' : 'dl',
'icontag' => $html5 ? 'div' : 'dt',
'captiontag' => $html5 ? 'figcaption' : 'dd',
'columns' => 3,
'size' => 'thumbnail',
'include' => '',
'exclude' => '',
'link' => '',
),
$attr,
'gallery'
);
$id = intval( $atts['id'] );
if ( ! empty( $atts['include'] ) ) {
$_attachments = get_posts(
array(
'include' => $atts['include'],
'post_status' => 'inherit',
'post_type' => 'attachment',
'post_mime_type' => 'image',
'order' => $atts['order'],
'orderby' => $atts['orderby'],
)
);
$attachments = array();
foreach ( $_attachments as $key => $val ) {
$attachments[ $val->ID ] = $_attachments[ $key ];
}
} elseif ( ! empty( $atts['exclude'] ) ) {
$attachments = get_children(
array(
'post_parent' => $id,
'exclude' => $atts['exclude'],
'post_status' => 'inherit',
'post_type' => 'attachment',
'post_mime_type' => 'image',
'order' => $atts['order'],
'orderby' => $atts['orderby'],
)
);
} else {
$attachments = get_children(
array(
'post_parent' => $id,
'post_status' => 'inherit',
'post_type' => 'attachment',
'post_mime_type' => 'image',
'order' => $atts['order'],
'orderby' => $atts['orderby'],
)
);
}
if ( empty( $attachments ) ) {
return '';
}
if ( is_feed() ) {
$output = "\n";
foreach ( $attachments as $att_id => $attachment ) {
$output .= wp_get_attachment_link( $att_id, $atts['size'], true ) . "\n";
}
return $output;
}
$itemtag = tag_escape( $atts['itemtag'] );
$captiontag = tag_escape( $atts['captiontag'] );
$icontag = tag_escape( $atts['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( $atts['columns'] );
$itemwidth = $columns > 0 ? floor( 100 / $columns ) : 100;
$float = is_rtl() ? 'right' : 'left';
$selector = "gallery-{$instance}";
$gallery_style = '';
/**
* Filters whether to print default gallery styles.
*
* @since 3.1.0
*
* @param bool $print Whether to print default gallery styles.
* Defaults to false if the theme supports HTML5 galleries.
* Otherwise, defaults to true.
*/
if ( apply_filters( 'use_default_gallery_style', ! $html5 ) ) {
$gallery_style = "
<style type='text/css'>
#{$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>\n\t\t";
}
$size_class = sanitize_html_class( $atts['size'] );
$gallery_div = "<div id='$selector' class='gallery galleryid-{$id} gallery-columns-{$columns} gallery-size-{$size_class}'>";
/**
* Filters the default gallery shortcode CSS styles.
*
* @since 2.5.0
*
* @param string $gallery_style Default CSS styles and opening HTML div container
* for the gallery shortcode output.
*/
$output = apply_filters( 'gallery_style', $gallery_style . $gallery_div );
$i = 0;
foreach ( $attachments as $id => $attachment ) {
$attr = ( trim( $attachment->post_excerpt ) ) ? array( 'aria-describedby' => "$selector-$id" ) : '';
if ( ! empty( $atts['link'] ) && 'file' === $atts['link'] ) {
$image_output = wp_get_attachment_link( $id, $atts['size'], false, false, false, $attr );
} elseif ( ! empty( $atts['link'] ) && 'none' === $atts['link'] ) {
$image_output = wp_get_attachment_image( $id, $atts['size'], false, $attr );
} else {
$image_output = wp_get_attachment_link( $id, $atts['size'], true, false, false, $attr );
}
$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' id='$selector-$id'>
" . wptexturize( $attachment->post_excerpt ) . "
</{$captiontag}>";
}
$output .= "</{$itemtag}>";
if ( ! $html5 && $columns > 0 && ++$i % $columns == 0 ) {
$output .= '<br style="clear: both" />';
}
}
if ( ! $html5 && $columns > 0 && $i % $columns !== 0 ) {
$output .= "
<br style='clear: both' />";
}
$output .= "
</div>\n";
return $output;
}
add_filter( 'post_gallery', 'my_custom_gallery', 10, 3 );