Help needed with SOAP and PHP 5

Hi Experts,

I have started learning PHP (version 5 )and SOAP.
I am able to connect to the setup a connection and got the output of
__getFunctions() method.
When I am trying to call a function defined there which needs
authentication and request in the following format,I am getting
errors.

<soapenv:Body>
<savePurchaseOrder
xmlns=”http://www.smartturn.com/services/OccamService/purchase-order”>
<inCredential>
<ns1:UserId
xmlns:ns1=”http://www.smartturn.com/services/occamtypes”>testuser</ns1:Us
erId>
<ns2:Password
xmlns:ns2=”http://www.smartturn.com/services/occamtypes”>testpassword</ns2:Password>

I dont know how to pass the value of namespace
(http://www.smartturn.com/services/OccamService/purchase-order)
and credential information through __soapCall() function.

Please let me know how to do this.

Thanks in advance .

-- 
Regards
Nirmal