如何转储SoapClient请求进行调试？

I need to debug a code wich use soap client. I found getLast* methods in php.net, but when I try to get last request for debug it returns NULL

<?php

    $client = new SoapClient("http://www.webservicex.net/ConverPower.asmx?WSDL");

    $response = $client->ChangePowerUnit(array(
        "PowerValue" => 100,
        "fromPowerUnit" => "horsepower",
        "toPowerUnit" => "megawatts"
    ));


    echo "====== REQUEST HEADERS =====" . PHP_EOL;
    var_dump($client->__getLastRequestHeaders());
    echo "========= REQUEST ==========" . PHP_EOL;
    var_dump($client->__getLastRequest());
    echo "========= RESPONSE =========" . PHP_EOL;
    var_dump($response);

?>

The result of code execution:

$php soap_test.php 

====== REQUEST HEADERS =====
NULL
========= REQUEST ==========
NULL
========= RESPONSE =========
object(stdClass)#2 (1) {
  ["ChangePowerUnitResult"]=>
  float(0.0746)
}

How to get the content of body and headers of the last SoapClient request?