[WordPress] Display gallery from Flickr album with simple shortcode

wp-flickr-album-shortcode.php

<?php
/**
 * Flickr Gallery shortcode
 *
 * @author Damian Wajer
 * @param $atts
 * @return string
 */
function prefix_flickr_gallery_shortcode( $atts ) {
	$a = shortcode_atts( array(
		'id'     => '',
		'width'  => '640',
		'height' => '480',
		'user'   => 'flickr',
	), $atts );

	if ( ! empty( $a['id'] ) ) {
		$url = 'https://www.flickr.com/photos/' . esc_attr( $a['user'] ) .'/sets/' . absint( $a['id'] ) .'/player/';

		$output = '<div class="embed-responsive embed-responsive-4by3">';
		$output .= '<iframe class="embed-responsive-item" src="' . esc_url( $url ) .'" width="' . absint( $a['width'] ) . '" height="' . absint( $a['height'] ) . '" frameborder="0" allowfullscreen webkitallowfullscreen mozallowfullscreen oallowfullscreen msallowfullscreen></iframe>';
		$output .= '</div>';

		return $output;
	}
}

add_shortcode( 'flickr', 'prefix_flickr_gallery_shortcode' );