code: public function getAuthorizationHeader($apiKey="")
      {
          if (empty($apiKey)) {
              if (empty($this->_apiKey)) throw new GreenhouseServiceException('No key provided to encode.');
              $apiKey = $this->_apiKey;
          }
          $key = rtrim($apiKey, ':') . ':';
          
          return 'Basic ' . base64_encode($key);
      }
docs: Base64 Encode an API key.  Greenhouse's encoding treats the key as the username with
  a blank password.  Here we append the : for convenience.  We also handle the case
  where the user appends the : themselves.  If no key is provided, we will attempt
  to encode the private api key property.
  
  @params  string  $apiKey     A greenhouse job board API key.
  @return  string  The other side of an Authorization header (Basic: <encoded key>)