code: public function setContentType($contentType = null)
      {
          if ($contentType !== null && !is_string($contentType)) {
              throw new Exception('Content-type value must be a valid string or null to use Soap verion defaults.');
          }
          $this->contentType = $contentType;
          return $this;
      }
docs: Sets the value of the contentType variable used in the content-type
  request header. Suggested values: text/xml; application/soap+xml;
  type/application-xml
  
  @param string $contentType
  @return $this
  @throws Exception