Force WordPress `srcset` to HTTPS

ssl_srcset.php

/*
 * Force URLs in srcset attributes into HTTPS scheme.
 * This is particularly useful when you're running a Flexible SSL frontend like Cloudflare
 */
function ssl_srcset( $sources ) {
  foreach ( $sources as &$source ) {
    $source['url'] = set_url_scheme( $source['url'], 'https' );
  }

  return $sources;
}
add_filter( 'wp_calculate_image_srcset', 'ssl_srcset' );