title: Call to undefined function get_field_id() in wordpress
body: <p>Fatal error: Call to undefined function get_field_id() in E:\wamp\www\customfield\wp-content\plugins\custom-widget2\update.php on line 6.</p>
  
  <ol>
  <li>i am using ajax to create second dropdown. my ajax page name is update.php but in response give me this error Call to undefined function get_field_id().</li>
  <li><p>i making a plugin for widgit </p>
  
  <p>
  
  <pre><code>        echo $ret='&lt;select id="'.get_field_id('test2').';" name=""&gt;';
  
               $categories=  get_categories();
                $categoryid="";
                $categoryid=5;
                $taxonomy_names = get_object_taxonomies($status);
                $taxonomy_name  =$taxonomy_names[0];
                $tax_terms = get_terms($taxonomy_name);
                foreach ($tax_terms as $tax_term):
                option = '&lt;option value="' .$tax_term-&gt;name . '" ';
  
                if ($cat-&gt;cat_ID == $categoryid) :
                    $option .= ' selected = "selected" ';
                endif;
                   $option .=  '"&gt;';
                   $option .= $tax_term-&gt;name;
                   $option .= '&lt;/option&gt;';
                   $ret.=$option;
                   echo $option;
               endforeach;
          $ret.="&lt;/select&gt;'";
  </code></pre>
  
  <p>update.php</p></li>
  </ol>
  
  <p>please help me......</p>