Note that calling __soapCall and calling the generated method from WSDL requires specifying parameters in two different ways.

For example, if you have a web service with method login that takes username and password, you can call it the following way:
<?php
$params = array('username'=>'name', 'password'=>'secret');
$client->login($params);
?>

If you want to call __soapCall, you must wrap the arguments in another array as follows:
<?php
$client->__soapCall('login', array($params));
?>

Sharing my experience cuz i believe it is most important for you if you decide to use this Soap Client implementation.

At php 7.0.8 the stdClass generated by SoapClient from the response does not use "minOccurs" and "maxOccurs" WSDL modifiers to distinct properties in stdClass-es (aka keys in "associative arrays") and elements in arrays ("aka indexed arrays"). 

Instead, the implementation decides whether tag is _a key in associative array_ or _one of elements with same tag in indexed array_ by simply the fact whether there is just one element with such tag in the sequence or many.

Consider alive example from my case:
<?php
response xml:
...
<ValidatingCarrier>
    <Alternate>AA</Alternate>
</ValidatingCarrier>
...
<ValidatingCarrier>
    <Alternate>AA</Alternate>
    <Alternate>AY</Alternate>
</ValidatingCarrier>
...

response structure generated by SoapClient:
...
[ValidatingCarrier] => stdClass Object(
    [Alternate] => AA // here it is a string
)
...
[ValidatingCarrier] => stdClass Object (
    [Alternate] => Array ( // and here this is an array
        [0] => AA
        [1] => AY
    )
)
...

field XSD definition:
<xs:element name="Alternate" type="CarrierCode" minOccurs="0" maxOccurs="24">
?>

You see, the definition in XSD tells us this tag can be repeated up to 24 times, what means it would be an indexed array, but SoapClient does not take that into account and treats the first <Alternate> in example as a value instead of array containing this value. 

Undoubtedly, a value should be a property in the stdClass (a key in associative array) _only_ when maxOccurs is 1 or less or not specified (default is 1, see https://www.w3.org/TR/xmlschema-0/#OccurrenceConstraints). Hope this info will be useful for you when you will be implementing your own, correctly working, SoapClient.

One thing to note.

This happened to me and it took a while until I discovered what the problem was.

I was trying to get .NET objects from a provided web service, however it always seemed to return empty objects. It did return the backbone, but nothing within the objects that made up the structure.

Anyhow, it seems that you have to be very precise with the arrays when calling these functions. Par example, do this:

<?php
$obj = $client->__soapCall($SOAPCall, array('parameters'=>$SoapCallParameters));
?>

meaning that you must put an array as the second argument with 'parameters' as the key and the soap call parameters as the value.

Also make sure that the parameter variable, in my case $SoapCallParameters is in the form of what is requested by the webservice.

So, don't just make an array of the form:
<?php

(
   [0] => 'Mary',
   [1] => 1983
)

?>

but if the webservice requests a 'muid' variable as 'Mary' and a 'birthyear' as 1983, then make your array like this:

<?php

(
   [muid] => 'Mary',
   [birthyear] => 1983
)

?>

The above arrays refer to the $SoapCallParameters variable.

Hope this helps somebody, not having to spend too much time figuring out the problems.

Following OrionI's example:

<?php
 $client = new SoapClient("http://server/sumservice.asmx?WSDL");
 $params->a = 2;
 $params->b = 3;
 $objectresult = $client->Sum($params);
 $simpleresult = $objectresult->SumResult;
 print($simpleresult); //produces "-1"
?> 

Please note that the lines: 

"$client->Sum($params);" 

and

"$simpleresult = $objectresult->SumResult;"

are based off of each other. If your web service function is called "Sum", then add "Result" to the end of it to get the results of the call.

EG:

<?php
 $client = new SoapClient("http://server/mathservice.asmx?WSDL");
 $params->a = 2;
 $params->b = 3;
 $objectresult = $client->Minus($params); // note the name of the function is "Minus"
 $simpleresult = $objectresult->MinusResult; // note the name of the result is referenced as "MinusResult"
 print($simpleresult); //produces "5"
?>

To avoid the SOAP client returning sometimes objects and sometimes arrays of objects, there's a setting:

   $this->soapClient = new \SoapClient($wsdlUrl, array(
    'features' => SOAP_SINGLE_ELEMENT_ARRAYS,
    'trace' => DEBUG_SOAP
   ));

I was working with SOAP for the first time and I had to create a client that sent a date range to a WSDL (Web Services Description Language) to return some information I needed.  I didn't know how to pass the params and there really was no documentation about it.  The main thing you have to make sure to do is when you pass params to a method that is definied by the WSDL that you are calling is that you use the same param name for the key of the array or the object variable as shown below.  Also, if you don't know what the methods/functions that a WSDL has or the params that you need to pass it you can use the __getFunctions() and __getTypes() methods after you declare your new SoapClient.

<?php
// From and to are the two params that the execute function needs
// when called from the WSDL so make you to have them as the
// key to an array like below
$params["From"] = "06/01/2005"; // also can use $params->From = "date";
$params["to"] = "12/31/2005"; // also can use $params->to = "date";

$client = new SoapClient("some.wsdl");

try {
        print($client->execute($params));
} catch (SoapFault $exception) {
        echo $exception;
}
?>

When calling over SOAP to a .NET application, you may end up with an object as a result instead of a simple type, even if you're just grabbing a simple type (like a boolean result). Use a property accessor to get the actual result, like this:
<?php
 $client = new SoapClient("http://server/myservice.asmx?WSDL");
 $objectresult = $client->MyMethod($param1, $param2);
 $simpleresult = $objectresult->MyMethodResult;
?>
Note that .NET seems to name the result MethodNameResult for method MethodName.

I am using SOAP call response headers to sign request results.

After alot of hours, I finally got the best way to get SOAP response headers (other than parsing __getLastResponse() witch requires trace option enabled) is using __soapCall() wrapper.

You can extend SoapClient class and wrap some functions to make sure you get the headers.

<?php

class API extends SoapClient
{

    // ... Constructor, etc.

    // Get SOAP Server response headers
    public function __soapCall($function, $arguments, $options = array(), $input_headers = null, &$output_headers = null)
    {
        parent::__soapCall($function, $arguments, $options, $input_headers, $output_headers);

        print_r($output_headers); // Array of response headers
    }

    // If you are using WSDL you need this, so you still can call functions directly without calling __soapCall manualy
    public function __call($func, $args)
    {
        return $this->__soapCall($func, $args);
    }

?>

Correction on the previously submitted code snippet...the incoming parameter for .NET also has to be in object or array form for it to be correctly converted to the XML form that .NET expects (as already mentioned by Llu?s P?mies). The full example (when using WSDL) should be like this:
<?php
 $client = new SoapClient("http://server/myservice.asmx?WSDL");
 $params->param1 = $value1;
 $params->param2 = $value2;
 $objectresult = $client->MyMethod($params);
 $simpleresult = $objectresult->MyMethodResult;
?>
So if you have a C# function like this:
//sumservice.asmx
...
[WebMethod]
public int Sum(int a, int b)
{
  return a + b;
}
...
The PHP client would be this:
<?php
 $client = new SoapClient("http://server/sumservice.asmx?WSDL");
 $params->a = 2;
 $params->b = 3;
 $objectresult = $client->Sum($params);
 $simpleresult = $objectresult->SumResult;
 print($simpleresult); //produces "5"
?>

One little gotcha when passing the parameters where you need to have attributes and a simpletype value: 

To get the xml

<foo bar="moo">cheese</foo>

You'd pass in:

<?php
array("foo" => array("_" => "cheese", "bar"=>"moo"));
?>

See that "_" bit? It really wasn't obvious from the documentation.

If you are using this method, remember that the array of arguments need to be passed in with the ordering being the same order that the SOAP endpoint expects.

e.g
<?php
//server expects: Foo(string name, int age)

//won't work
$args = array(32, 'john');
$out = $client->__soapCall('Foo', $args);

//will work
$args = array('john', 32);
$out = $client->__soapCall('Foo', $args);
?>

The call to __soapCall returned an object to me either. This is the function which makes my life easier:

function obj2array($obj) {
  $out = array();
  foreach ($obj as $key => $val) {
    switch(true) {
        case is_object($val):
         $out[$key] = obj2array($val);
         break;
      case is_array($val):
         $out[$key] = obj2array($val);
         break;
      default:
        $out[$key] = $val;
    }
  }
  return $out;
}

Usage:
...
$response = $client ->__soapCall("track", array('parameters' => $request));
$response = obj2array($response);

Hope it helps.