Error exception: Class “SoapClient” not found

class MbeSoapClient extends SoapClient {    
public $isOnlinembe = false;    
private $helper;
public function __construct( $wsdl, array $options = null, $username = null, $password = null, $wsdlcache = true)

mail-boxes-etc/lib/Mbe/MbeSoapClient.php:3