code: protected function parseHeader(array $headers = [])
      {
          $processedHeaders = array();
          foreach ($headers as $key => $value) {
              if (substr($key, 0, 5) == 'HTTP_') {
                  $name = str_replace(' ', '-', ucwords(strtolower(str_replace('_', ' ', substr($key, 5)))));
                  $processedHeaders[$name] = $value;
              } elseif ($key == 'CONTENT_TYPE') {
                  $processedHeaders['Content-Type'] = $value;
              } elseif ($key == 'CONTENT_LENGTH') {
                  $processedHeaders['Content-Length'] = $value;
              }
          }
          return $processedHeaders;
      }
docs: Parse headers.
  
  This method should be passed the _SERVER global array. Most http headers
  in PHP are prefixed with HTTP_ however, there are two values prefixed
  with CONTENT_. All other values in the array will be ignored.
  
  @param string[] $headers
  @return string