/

I have a similar issue. I present you a complete test case:

<?php

/** Function that is the culprit **/
function filter($o)
{
	if(is_array($o))
	{
		foreach($o as &$v)
		{
			$v = filter($v);
		}
		
		return $o;
	}
	
	return $o;
}

/**
 * Test class
 */
class TestClient extends SoapClient
{

	public function __construct()
	{
		parent::__construct('http://www.ripedev.com/webservices/localtime.asmx?WSDL');
	}

	public function __doRequest($request, $location, $action, $version, $one_way = NULL)
	{
		echo htmlentities($request);
		die();
	}
}

$args = array(
	"ZipCode" => null 
);

/**
 * When commentend (ZipCode not there):
 * <?xml version="1.0" encoding="UTF-8"?>
 * <SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ns1="http://www.ripedev.com/">
 * <SOAP-ENV:Body>
 * <ns1:LocalTimeByZipCode/>
 * </SOAP-ENV:Body>
 * </SOAP-ENV:Envelope>
 *
 * When uncommented (zipCode is now an empty string??):
 * <?xml version="1.0" encoding="UTF-8"?>
 * <SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ns1="http://www.ripedev.com/">
 * <SOAP-ENV:Body>
 * <ns1:LocalTimeByZipCode>
 * <ns1:ZipCode></ns1:ZipCode>
 * </ns1:LocalTimeByZipCode>
 * </SOAP-ENV:Body>
 * </SOAP-ENV:Envelope>
 */
// $args = filter($args);

// Both show same var_dump:

// var_dump($args);
// array(1) { ["ZipCode"]=> NULL }
// var_dump(filter($args));
// array(1) { ["ZipCode"]=> NULL }

$client = new TestClient();
$client->LocalTimeByZipCode($args);

We are also having a same problem, but with in SoapServer context. If a variable (array of objects) has a reference the soap action will return an array of empty elements. See the example below. When the reference is removed,  I don't know either if this is soap related, but I was not able to reproduce this with a separate test script. 

I think this is a really critical bug. This was working as it should in all versions before php7.




-------

public function getSalesOrders() {
  $data = [new SalesOrder()];

  foreach($data as &$item) { // ref here breaks the response.
    $data->rows = getRows($item->id);
  }

  return $data;
}
--------
<SOAP-ENV:Envelope>
   <SOAP-ENV:Body>
      <ns1:getSalesOrdersResponse>
         <getSalesOrdersReturn SOAP-ENC:arrayType="ns1:SalesOrder[3]" xsi:type="ns1:SalesOrderArray">
            <item xsi:type="ns1:SalesOrder"/>
            <item xsi:type="ns1:SalesOrder"/>
            <item xsi:type="ns1:SalesOrder"/>
         </getSalesOrdersReturn>
      </ns1:getSalesOrdersResponse>
   </SOAP-ENV:Body>
</SOAP-ENV:Envelope>


-------

And yes, I know that the reference is not needed in my case, but there might be cases where it was needed. This was just something we noticed when upgrading to php7.

Correction to previous code example. It should of course be:
  $item->rows = getRows($item->id);

Duplicate of bug #73089, which should be fixed in either 7.0.12 or 7.0.15 depending on the particular case.