How to override SoapClient::__doRequest()?

Question

I want to override SoapClient::__doRequest() in my custom class and use my own curl based transport, as suggested here.

According to the PHP manual, __doRequest() has this signature:

public string SoapClient::__doRequest ( string $request , string $location , string $action , int $version [, int $one_way = 0 ] )

And the $request parameter is:

• request The XML SOAP request.

But this seems to be wrong. I don't get any XML in $request but a cryptic string like:

string(1496) " 14120188025349F

Here's the simple class I've used:

class MySoapClient extends SoapClient
{
    public function __doRequest($request, $location, $action, $version, $one_way = false)
    {
        var_dump($request);
        return parent::__doRequest($request, $location, $action, $version, $one_way);
    }
}

So what is wrong? Did they somehow change the internal implementation of this without updating the manual? For reference, my PHP version is 5.4.6 on a Ubuntu 12.10 box.