code: protected function prepareForNextMessage($messageName, $messageOptions)
      {
          if ($this->isAuthenticated === true && is_int($this->sessionData['sequenceNumber'])) {
              $this->sessionData['sequenceNumber']++;
          }
  
          $headers = $this->createSoapHeaders($this->sessionData, $this->params, $messageName, $messageOptions);
  
          $this->getSoapClient($messageName)->__setSoapHeaders(null);
          $this->getSoapClient($messageName)->__setSoapHeaders($headers);
      }
docs: Handles authentication & sessions
  
  If authenticated, increment sequence number for next message and set session info to soapheader
  If not, set auth info to soapheader
  
  @uses $this->isAuthenticated
  @uses $this->sessionData
  @param string $messageName
  @param array $messageOptions