responsive-back.php

// Add responsive background image code
function responsive_bg_styles($attachment_id, $selector) {
  $srcset = wp_get_attachment_image_srcset($attachment_id);
  $sets   = explode(', ', $srcset);

  if (is_array($sets)) {
    
    $set_build = array();
    
    foreach ($sets as $set) {
      list($url, $width) = explode(' ', $set);
      $width = str_replace('w', '', $width);
      $set_build[$url] = $width;
    }

    asort( $set_build );
    $min_width = 0;
       
    echo '<style>';
      foreach ($set_build as $key => &$val) { ?>
        
        @media (min-width: <?php echo $min_width ?>px) {
          <?php echo $selector; ?> {
            background-image: url(<?php echo $key; ?>);
          }
        }
        <?php $min_width = $val+1;
      }
    echo '</style>';
  }
}