src/Core/ServiceContext.php
public function useXml()
    {
        $this->IppConfiguration->Message->Request->CompressionFormat = CompressionFormat::None;
        $this->IppConfiguration->Message->Response->CompressionFormat = CompressionFormat::None;
        $this->IppConfiguration->Message->Request->SerializationFormat = SerializationFormat::Xml;
        $this->IppConfiguration->Message->Response->SerializationFormat = SerializationFormat::Xml;
    }
public function useXml()
    {
        $this->IppConfiguration->Message->Request->CompressionFormat = CompressionFormat::None;
        $this->IppConfiguration->Message->Response->CompressionFormat = CompressionFormat::None;
        $this->IppConfiguration->Message->Request->SerializationFormat = SerializationFormat::Xml;
        $this->IppConfiguration->Message->Response->SerializationFormat = SerializationFormat::Xml;
    }
Currently the Object serailziation and deserilization only supports XML format.