code: public static function getResponseHeader($key = 'Content-Type')
      {
          $value = null;
          $headers = \headers_list();
          foreach ($headers as $header) {
              if (\preg_match('/^'.$key.':\s*([^;]*)/i', $header, $matches)) {
                  $value = $matches[1];
                  break;
              }
          }
          return $value;
      }
docs: Returns a sent/pending response header value
  
  @param string $key default = 'Content-Type', header to return
  
  @return string
  @req    php >= 5