Note when extending __doRequest, calling __getLastRequest will probably report incorrect information unless you make sure to update the internal __last_request variable. Save yourself some headaches.

function __doRequest($request, $location, $action, $version) {
      $request = preg_replace('/abc/', 'def', $request);
      $ret = parent::__doRequest($request, $location, $action, $version);
      $this->__last_request = $request;
      return $ret;
}

I just wanted to note in plain english how to build up this request, because I made some assumptions by just misterpreting the parameters.

This is for future reference(for myself) and others that might end up a bit mistified by just not understanding the finesses of soap and are trying to learn.

    $this->__doRequest(string $request , string $location , string $action , int $version [, int $one_way = 0 ] );

$request = The XML Soap enveloppe
$location = The url to the WSDL file. No matter that you defined this before when setting up the object, you need to reuse it here.
$action = the soap action to be performed. This is defined in the wsdl file and can be in the form of a single form or an url. it's just a parameter and might not be an actual valid url
$version = SOAP_1_1 =  content headers (Content-Type: text/xml; charset=utf-8␍)
                  SOAP_1_2 = content headers (Content-Type: application/soap+xml; charset=utf-8; action="somesoapaction defined in $action")

If you send a SOAP_1_2 request to a SOAP_1_1 server you might get a reply in the following form 

    HTTP/1.1 415 Cannot process the message because the content type 'application/soap+xml; charset=utf-8; action="somesoapaction defined in $action"' was not the expected type 'text/xml; charset=utf-8'.

You need to switch down to SOAP_1_1 in that case to get the proper formatting the server can understand

Note that the SoapClient.__doRequest() method circumvents the throwing of SoapFault exceptions.

Specifically, if you call the __doRequest() method and it fails, it would normally throw a SoapFault exception.  However, the __doRequest() method doesn't actually throw the exception. Instead, the exception is saved in a class attribute called SoapFault.__soap_fault, and is actually thrown AFTER the __doRequest method completes (but the call stack will show that the exception was created inside the __doRequest method.

I successfully used the following code to query the locally cached exception object that was not thrown:

<?php
$exception = null;
try {
    $result = parent::__doRequest($request, $location, $action, $version, $one_way);
}
catch (SoapFault $sf) {
    //this code was not reached    
    $exception = $sf;
}
catch (Exception $e) {
    //nor was this code reached either
    $exception = $e;
}
if((isset($this->__soap_fault)) && ($this->__soap_fault != null)) {
    //this is where the exception from __doRequest is stored
    $exception = $this->__soap_fault;
}

//decide what to do about the exception here
// [enter code here]
//or throw the exception
if($exception != null) {
    throw $exception;
}
//note: you may want to unset the __soap_fault value if you don't want it thrown again up the call stack
?>

If you want to communicate with a default configured ASP.NET server with SOAP 1.1 support, override your __doRequest with the following code. Adjust the namespace parameter, and all is good to go.

<?php
class MSSoapClient extends SoapClient {

    function __doRequest($request, $location, $action, $version) {
        $namespace = "http://tempuri.com";

        $request = preg_replace('/<ns1:(\w+)/', '<$1 xmlns="'.$namespace.'"', $request, 1);
        $request = preg_replace('/<ns1:(\w+)/', '<$1', $request);
        $request = str_replace(array('/ns1:', 'xmlns:ns1="'.$namespace.'"'), array('/', ''), $request);

        // parent call
        return parent::__doRequest($request, $location, $action, $version);
    }
}

$client = new MSSoapClient(...);
?>

Hope this will save people endless hours of fiddling...

Note that __getLastRequest() data are buffered _before_ the call to __doRequest().  Thus any modifications you make to the XML while in __doRequest() will not be visible in the output of __getLastRequest().   This is so in at least v5.2.0

If you happen to get an error during your request which says "SOAP-ERROR: Encoding: Can't decode apache map, only Strings or Longs are allowd as keys", the reason seems to be the response xml using integer for keys and php failling to understand them

Here is something that worked for me (converts integer keys to strings):

<?php
class mySoap extends SoapClient
{
    public function __doRequest($request, $location, $action, $version)
    {
        $result = parent::__doRequest($request, $location, $action, $version);
        $result = str_replace('<key xsi:type="xsd:int">', '<key xsi:type="xsd:string">', $result);
        return $result;
    }
}

// $soap = new mySoap(...
?>

If you're having trouble connecting to an authenticated SOAP service there's something important to note here.

__doRequest() is only used when a SOAPClient function is called, NOT when the WSDL if fetched and parsed.  That means if your WSDL file is not publicly accessible but also resides behind your authentication it won't be accessible by default.  Instead you'll have to create an overloaded stream wrapper and register it for whatever protocol (probably HTTP) you'll be using.

Beware of PHP incosistent behaviour in __doRequest() method. It seems that some arguments passed to this method are passed by reference! 

If you try to create your own __doRequest() method and store it's arguments as SoapClient properties you will find that after __soapCall all of them will be null or unknown.

<?php
    protected $__soapAction = '';

    public function __doRequest($request, $location, $action, $version, $oneWay = 0) {
        ob_start();
        $this->server->handle($request);
        $response = ob_get_contents();
        ob_end_clean();
        $this->__soapAction = $action;
        return $response;
    }
?>

In above example $this->__soapAction will be null after $obj->__soapCall()..

To store $action value, you must cast it to a string (so PHP will be forced to create a new variable with different memory pointer):

<?php
    public function __doRequest($request, $location, $action, $version, $oneWay = 0) {
        ob_start();
        $this->server->handle($request);
        $response = ob_get_contents();
        ob_end_clean();
        $this->__soapAction = (string)$action;
        return $response;
    }
?>

I was needing to add a blank soap header (<SOAP-ENV:Header /> and found no other place that has done this.  The only way I was able to support this was to extend SoapClient and re-define __doRequest.  Hope this helps someone or if there is support for this in the library, please point me in the right direction:

<?php
class MySoapCli extends SoapClient {
  function __doRequest($request, $location, $action, $version) {
    $dom = new DomDocument('1.0', 'UTF-8');
    $dom->preserveWhiteSpace = false;
    $dom->loadXML($request);
    $hdr = $dom->createElementNS('http://schemas.xmlsoap.org/soap/envelope/', 'SOAP-ENV:Header');
    $dom->documentElement->insertBefore($hdr, $dom->documentElement->firstChild);
    $request = $dom->saveXML();
    return parent::__doRequest($request, $location, $action, $version);
  }
}
?>

Change Blocking mode for Socket stream and set Timeout for Soap requests

<?php

class TimeoutSoapClient extends SoapClient
{
    const TIMEOUT = 20;
    public function __doRequest($request, $location, $action, $version, $one_way = 0)
    {
        $url_parts = parse_url($location);
        $host = $url_parts['host'];
        $http_req = 'POST '.$location.' HTTP/1.0'."\r\n";
        $http_req .= 'Host: '.$host."\r\n";
        $http_req .= 'SoapAction: '.$action."\r\n";
        $http_req .= "\r\n";
        $http_req .= $request;
        $port = 80;
        if ($url_parts['scheme'] == 'https')
        {
            $port = 443;
            $host = 'ssl://'.$host;
        }
        $socket = fsockopen($host, $port);
        fwrite($socket, $request);
        stream_set_blocking($socket, false);
        $response = '';
        $stop = microtime(true) + self::TIMEOUT;
        while (!feof($socket))
        {
            $response .= fread($socket, 2000);
            if (microtime(true) > $stop)
            {
                throw new SoapFault('Client', 'HTTP timeout');
            }
        }
        return $response;
    }
}

You can use this method to correct the SOAP request before sending it, if necessary. You can use the DOM API to accomplish that. 

<?php

public ExtendedClient extends SoapClient {

   function __construct($wsdl, $options = null) {
      parent::__construct($wsdl, $options);
   }

   function __doRequest($request, $location, $action, $version) {
      $dom = new DOMDocument('1.0');

      try {

         //loads the SOAP request to the Document
         $dom->loadXML($request);

      } catch (DOMException $e) {
         die('Parse error with code ' . $e->code);
      }

      //create a XPath object to query the request
      $path = new DOMXPath($dom);

      //search for a node
      $nodesToFix = $path->query('//SOAP-ENV:Envelope/SOAP-ENV:Body/path/to/node');

      //check if nodes are ok
      $this->checkNodes($path, $nodesToFix);

      //save the modified SOAP request
      $request = $dom->saveXML();
      
      //doRequest
      return parent::__doRequest($request, $location, $action, $version);
   }

   function checkNodes(DOMXPath $path, DOMNodeList $nodes) {
      //iterate through the node list
      for ($i = 0; $ < $nodes->length; $i++) {
         $aNode = $nodes->item($i);

         //just an example
         if ($node->nodeValue == null) { 
            //do something. For instance, let's remove it.
            $node->parentNode->removeChild($node);
         }
      }
   }
}
?>

This gives the developer the chance to solve interoperability problems with a web service.

Do you have problems with the PHP5 SoapClient when you need to send a request to a service with a ComplexType parameter?
 
Maybe because my service is build in Delphi with REMObjects SDK 3.0 I had the problems, maybe not. Anyway, this was my remedy:
<?php
$versie = new stdClass();//define a basic class object
$versie->versieID = $aVersie->versieID();//fill it with the exact attributes as your complextype Object in the wsdl is
$versie->versieNummer = $aVersie->versieNummer();
$versie->isActief = $aVersie->isActief();      
     
$soapVersieType = new SoapVar($versie , SOAP_ENC_OBJECT, "Versie", "http://127.0.0.1:8999/SOAP?wsdl"); //create the complex soap type, Versie is the name of my complex type in the wsdl, the latter url beeing the location of my wsdl.
 
try{
  $result =  $soapClient->BewaarVersie($this->sessieId,$soapVersieType); //BewaarVersie is a function derived from my WSDL with two params.
}
catch(SoapFault $e){
  trigger_error('Something soapy went wrong: '.$e->faultstring,E_USER_WARNING);            }
?>
 
After some more testing i found out that the conversion to the StdClass() object was not required. My 'Versie' local object has the attributes for the 'Versie' wsdl complex type defined as private vars and give no pain when i create the SoapVar with an instance of the local 'Versie' Object.

If your application interacts with SOAP services and you wish to cache the responses for consumption later, then overriding SoapClient::__doRequest is the way to go.

For instance, if you know that the information presented doesn't change that often and you don't want to do a superfluous HTTP request, you can grab a response from a local cache and let SoapClient do the transformation to PHP data types.

<?php
class YourNamespace_SoapClient_Local extends SoapClient {
    protected $cacheDocument = "";
    public function __construct($wsdl, $options) {
        parent::__construct($wsdl, $options);
    }

    /**
     * SetCacheDocument() sets the previously cached document contents
     */
    public function SetCacheDocument($document) {
        $this->cacheDocument = $document;
    }

    /**
     * __doRequest() overrides the standard SoapClient to handle a local request
     */
    public function __doRequest() {
        return $this->cacheDocument;
    }
}

//---- code snippet showing usage within a class
//$document is a cached SOAP response document from a previous request, saved with SoapClient::__getLastResponse() to some cache somewhere
//for the purpose of this example, it is assumed that $this->wsdl, $this->options, $this->method and $this->params are set.

public function SoapRequest($document) {
    $method = $this->method;
    if($document == "") {
        //uncached
        try {
            //default options
            $client = new SoapClient($this->wsdl, $this->options);
            $result = $client->$method($this->params);
            //send the response to the cache
            $this->CacheResponse($client->__getLastResponse());
        } catch(SoapFault $fault) {
            //log something
            return FALSE;
        }
    } else {
        //cached document
        try {
            /**
             * the WSDL needs to be set to allow the method to be called on the client object
             * and to trigger SoapClient to decode the response to native data types
             */
            $client = new YourNamespace_SoapClient_Local($this->wsdl, $this->options);
            $client->SetCacheDocument($document);
            $result = $client->$method($this->params);
        } catch (SoapFault $fault) {
            //log something
            return FALSE;
        }
    }
    return $result;
}
?>

I'll leave you to work out the caching, plenty of options there.. ;)