code: public function getHeader(string $name): ?string
      {
          $name = strtolower($name);
          foreach ($this->getAllHeaders() as $key => $value) {
              if (strtolower($key) === $name) {
                  return $value;
              }
          }
          return null;
      }
docs: It returns the header by passed name. The search is case-insensitive.
  @param string $name
  @return string|null