Description:
------------
Nested foreach loops using assign-by-reference produce variables which, when passed to SoapClient, cause errors. We discovered this bug when passing data returned by the official AWS DynamoDB client to SoapClient, but reproduce it below with a few lines of PHP.

https://github.com/aws/aws-sdk-php/blob/3.17.5/src/DynamoDb/Marshaler.php#L285

Test script:
---------------
<?php

$client = new SoapClient(null, ['uri' => '', 'location' => '']);

$data = [[null]];

foreach ($data as &$value) {
    foreach ($value as &$_value) {
        $_value = 'foo';
    }
}

$client->__soapCall('call', $data);

Expected result:
----------------
PHP Fatal error:  Uncaught SoapFault exception: [HTTP] Unable to parse URL

(Because SoapClient attempts to retrieve the WSDL from the invalid URL provided in the test code. This error is seen in PHP5.)

Actual result:
--------------
Fatal error: Uncaught SoapFault exception: [Client] SOAP-ERROR: Encoding: Cannot find encoding