SOAP-ERROR: Parsing WSDL: Unknown required WSDL extension 'http://schemas.xmlsoap.org/ws/2004/09/policy' in PHP SoapClient while calling SAP PI

I'm trying to implement a Web Service running on an SAP PI system. The WSDL I got looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<wsdl:definitions name="request_SO" targetNamespace="http://company.ch/orstest/jason_test" xmlns:p1="http://company.ch/orstest/jason_test" xmlns:wsp="http://schemas.xmlsoap.org/ws/2004/09/policy" xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd" xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/">
  <wsdl:documentation />
  <wsp:UsingPolicy wsdl:required="true" />
  <wsp:Policy wsu:Id="OP_request_SO" />
  <wsdl:types>
    <xsd:schema targetNamespace="http://company.ch/orstest/jason_test" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://company.ch/orstest/jason_test">
      <xsd:element name="request" type="request" />
      <xsd:element name="response" type="response" />
      <xsd:complexType name="request">
        <xsd:annotation>
          <xsd:appinfo source="http://sap.com/xi/VersionID">a8b7948ac2e211e2b2380000001f0d6a</xsd:appinfo>
        </xsd:annotation>
        <xsd:sequence>
          <xsd:element name="body" type="xsd:string" />
        </xsd:sequence>
      </xsd:complexType>
      <xsd:complexType name="response">
        <xsd:annotation>
          <xsd:appinfo source="http://sap.com/xi/VersionID">bf49af09c36c11e299790000001f0d6a</xsd:appinfo>
        </xsd:annotation>
        <xsd:sequence>
          <xsd:element name="body" type="xsd:string" />
        </xsd:sequence>
      </xsd:complexType>
    </xsd:schema>
  </wsdl:types>
  <wsdl:message name="request">
    <wsdl:documentation />
    <wsdl:part name="request" element="p1:request" />
  </wsdl:message>
  <wsdl:message name="response">
    <wsdl:documentation />
    <wsdl:part name="response" element="p1:response" />
  </wsdl:message>
  <wsdl:portType name="request_SO">
    <wsdl:documentation />
    <wsdl:operation name="request_SO">
      <wsdl:documentation />
      <wsp:Policy>
        <wsp:PolicyReference URI="#OP_request_SO" />
      </wsp:Policy>
      <wsdl:input message="p1:request" />
      <wsdl:output message="p1:response" />
    </wsdl:operation>
  </wsdl:portType>
  <wsdl:binding name="request_SOBinding" type="p1:request_SO">
    <soap:binding style="document" transport="http://schemas.xmlsoap.org/soap/http" xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/" />
    <wsdl:operation name="request_SO">
      <soap:operation soapAction="http://sap.com/xi/WebService/soap1.1" xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/" />
      <wsdl:input>
        <soap:body use="literal" xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/" />
      </wsdl:input>
      <wsdl:output>
        <soap:body use="literal" xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/" />
      </wsdl:output>
    </wsdl:operation>
  </wsdl:binding>
  <wsdl:service name="request_SOService">
    <wsdl:port name="HTTP_Port" binding="p1:request_SOBinding">
      <soap:address location="http://qa1100ap601.company.car.web:50000/XISOAPAdapter/MessageServlet?senderParty=&amp;senderService=BS_ORS&amp;receiverParty=&amp;receiverService=&amp;interface=request_SO&amp;interfaceNamespace=http://company.ch/orstest/jason_test" xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/" />
    </wsdl:port>
    <wsdl:port name="HTTPS_Port" binding="p1:request_SOBinding">
      <soap:address location="https://qas.soa.subcompany.company.ch/XISOAPAdapter/MessageServlet?senderParty=&amp;senderService=BS_ORS&amp;receiverParty=&amp;receiverService=&amp;interface=request_SO&amp;interfaceNamespace=http://company.ch/orstest/jason_test" xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/" />
    </wsdl:port>
  </wsdl:service>
</wsdl:definitions>

My PHP file looks like this (nothing more in it currently):

<?php 
$client = new SoapClient('test.wsdl');
?>

When I'm running my script, I get the following error thrown:

Fatal error: Uncaught SoapFault exception: [WSDL] SOAP-ERROR: Parsing WSDL: Unknown required WSDL extension 'http://schemas.xmlsoap.org/ws/2004/09/policy' in /srv/www/htdocs/wsdl.php:2 Stack trace: #0 /srv/www/htdocs/wsdl.php(2): SoapClient->SoapClient('test.wsdl') #1 {main} thrown in /srv/www/htdocs/wsdl.php on line 2

I tried searching for the error on Google and Stack Overflow, and also checked the comments on php.net, but I can't seem to figure out what I'd have to change in order to get this service running. I also tried to open the WSDL extension URL, it seems to exist.

Can someone throw me a bone here? I tried removing the extension URL, that results in the following error:

Fatal error: Uncaught SoapFault exception: [WSDL] SOAP-ERROR: Parsing WSDL: Unexpected WSDL element in /srv/www/htdocs/wsdl.php:2 Stack trace: #0 /srv/www/htdocs/wsdl.php(2): SoapClient->SoapClient('test.wsdl') #1 {main} thrown in /srv/www/htdocs/wsdl.php on line 2

The only solution I can think of at this point is to update your UsingPolicy tag:

Change the tag:

<wsp:UsingPolicy wsdl:required="true"/> 

to this:

<wsp:UsingPolicy wsdl:required="false"/>