there is no class using underscore in ZF2. use : 

    AbstractValue::getXmlRpcValue

instead. and use 

    AbstractValue::XML_STRING

instead. already at use statement : https://github.com/volftomas/zf2/blob/d645eb295cd99626b170f60a329321acd11419ba/tests/ZendTest/XmlRpc/ValueTest.php#L14