Output all images from gallery inside a slider

Ok, so, I found this function, but it wasn’t in the codex. It came from Pippin’s Plugins, just to clarify for any future readers.

It appears this function was used as a filter added onto the content, and will only run when filtering existing content, because it’s retrieving & replacing the galleries placed inside the post content.

So, I’d recommend adding the filter back on, and not using outside of the main content, here’s a reworked version of the function repurposed for your slider.

function pw_show_gallery_image_urls( $content ) {

    global $post;

    // Only do this on singular items
    if( ! is_singular() )
        return $content;

    // Make sure the post has a gallery in it
    if( ! has_shortcode( $post->post_content, 'gallery' ) )
        return $content;

    // Retrieve all galleries of this post
    $galleries = get_post_galleries_images( $post );

    // Loop through all galleries found
    foreach( $galleries as $gallery ) {

        $image_list = array();

        // Loop through each image in each gallery
        foreach( $gallery as $image ) {

            $image_list[] = '<div><img src="' . $image . '" ></div>';

        }

        // Append our image list to the content of our post
        if ( !empty( $image_list ) {

            $content .= '<div class="slider">' . implode('', $image_list) . '</div>';

        }

    }

    return $content;

}
add_filter( 'the_content', 'pw_show_gallery_image_urls' );

If this doesn’t work the way you want it to, though, you might have to find a different function to start from, or write it from scratch.