customizing wordpress gallery output

I want to modify the default wordpress gallery and add a button below each of the image inside gallery.

I was able to extract the code from wordpress media.php file and include it in my function.php file.
I need help with this part in the code:

//add a button here which will be like
$output .= <div class="custom-class" data-href="<url-of-image.png>">Like</div>;

Please help to write above in php.
here is the Code I have included in function.php file:

add_filter('post_gallery', 'my_post_gallery', 10, 2);
function my_post_gallery($output, $attr) {
    global $post;

$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}>";

//add a button here which will be like:
//$output .= "<div class="custom-class" data-href="<url-of-image.png>">Like</div>";

    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;
}