public function has(string $key): bool
    {
        $key = $this->getKey($key, true);
        return isset($this->headers[$key]);
    }
public function has(string $key): bool
    {
        $key = $this->getKey($key, true);
        return isset($this->headers[$key]);
    }
public function has ( string $ key ) : bool { $ key = $ this -> getKey ( $ key , true ) ; return isset ( $ this -> headers [ $ key ] ) ; }
Determine whether a particular header exists

@param string $key The key to check for
@return bool
Determine whether a particular header exists