I hope I am posting this in the right place.

I am working with a response, that is captured in a PHP variable (response from a cURL post). That response looks something like this.
Code:
<soapenv:Envelope xmlns:log="http://somesite.com" xmlns:soapenv="http://somesite.com/" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xsd="http://somesite.com">

<soapenv:Header>
<wssb:Security xmlns:wssb="http://somesite.com" xmlns:wsu="http://somesite.com">
<wsu:Timestamp wsu:Id="WS9ae008c3-a845-13a8-a6a0-dd227c7c567e">
<wsu:Created>2010-05-04T20:18:13Z</wsu:Created>
<wsu:Expires>2010-05-04T21:18:13Z</wsu:Expires>
</wsu:Timestamp>
<wssb:UsernameToken wsu:Id="abcd-efg-hijk">
<wssb:Username>username</wssb:Username>
<wssb:Password Type="wssb:PasswordDigest">1234ABCD=</wssb:Password>
<wssb:Nonce>AAbdEd==</wssb:Nonce>
<wsu:Created>2010-05-03T18:37:39Z</wsu:Created>
</wssb:UsernameToken>
</wssb:Security>
<log:customattrs/>
</soapenv:Header>
<soapenv:Body><xsd:getAccountOperationRequest/></soapenv:Body>
</soapenv:Envelope>
What I need to do is capture the element "wssb:Security". In other words, extract all of the information between: "<wssb:Security" and "</wssb:Security>".

What would be my best approach in doing so?
I am new to this, so a code example would be greatly appreciated along with any references.