Add anchor tag on previous_image_link / next_image_link

i’m usig the code for create a link (next and prev)

$attachments = array_values( get_children( array( 'post_parent' => $post->post_parent, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => 'ASC', 'orderby' => 'menu_order ID' ) ) );
foreach ( $attachments as $k => $attachment ) :
    if ( $attachment->ID == $post->ID )
        break;
endforeach;

$k++;
// If there is more than 1 attachment in a gallery
if ( count( $attachments ) > 1 ) :
    if ( isset( $attachments[ $k ] ) ) :
        // get the URL of the next image attachment
        $next_attachment_url = get_attachment_link( $attachments[ $k ]->ID );
    else :
        // or get the URL of the first image attachment
        $next_attachment_url = get_attachment_link( $attachments[ 0 ]->ID );
    endif;
else :
    // or, if there's only 1 image, get the URL of the image
    $next_attachment_url = wp_get_attachment_url();
endif;

for the ancor use the id a div and add a link that id

<div id="ancor">ancor top</div>

<a href="<?php echo esc_url( $prev_attachment_url ); ?>#ancor" title="<?php the_title_attribute(); ?>" rel="attachment"></a>