gistfile1.php

function lpg_get_attachment( $attachment_id ) {
  $attachment = get_post( $attachment_id );
  return array(
    'alt' => get_post_meta( $attachment->ID, '_wp_attachment_image_alt', true ),
    'caption' => $attachment->post_excerpt,
    'description' => $attachment->post_content,
    'href' => get_permalink( $attachment->ID ),
    'src' => $attachment->guid,
    'title' => $attachment->post_title
  );
}

/**
 * Generate responsive image
 *
 * Generates a responsive image tag (using srcset) for the given attachment ID
 * using the given thumbnail sizes matching the given media queries (sizes).
 * Default image will be the first size given.
 */
function lpg_srcset($id, $sizes, $mq, $classes) {
  $meta = lpg_get_attachment($id);
  $images = [];
  foreach ($sizes as $size) {
    $images[] = wp_get_attachment_image_src($id, $size);
  }

  return sprintf(
    '<img src="%s" sizes="%s" srcset="%s" alt="%s" width="%d" height="%d" class="%s">',
    '', //$images[0][0], // ** Source omitted for picturefill polyfill
    join($mq, ','),
    join(array_map(function ($x) { return $x[0] . ' ' . $x[1] . 'w'; }, $images), ','),
    $meta['alt'],
    $images[0][1],
    $images[0][2],
    join($classes, ' ')
  );
}