if ( empty( is_active_sidebar('sideby-post-widget') ) ) {
  $class_shop = 'fullw';
}else {
  $class_shop = '';
}
?>

    <div class="clomens">

      <br />
<b>Fatal error</b>:  Uncaught Error: Call to undefined function is_active_sidebar() in /home/rockyw/domains/rockyweld.com/public_html/index.php:19
Stack trace:
#0 {main}
  thrown in <b>/home/rockyw/domains/rockyweld.com/public_html/index.php</b> on line <b>19</b><br />