How to get namespace using prefix

Hello,
Could anyone let me know how to get the namespace if I already know the
prefix

My xml file is actually a WSDL. Please find below a portion of it

    - <wsdl:definitions targetNamespace="http://test" xmlns:impl="
    http://test" xmlns:intf="http://test" xmlns:wsdl="
    http://schemas.xmlsoap.org/wsdl/" xmlns:wsdlsoap="
    http://schemas.xmlsoap.org/wsdl/soap/" xmlns:xsd="
    http://www.w3.org/2001/XMLSchema">
      - <wsdl:binding name="ServiceSoapBinding" type="intf:Service">
          <wsdlsoap:binding style="document" transport="
        http://schemas.xmlsoap.org/soap/http" />
      </wsdl:binding>
</wsdl:definitions>

  At any point I would like to know how to get the namespace using its
  prefix.

  for eg: if I give 'intf' as the prefix how to know that "http://test" is
  the namespace.

  Thanks in advance,
  Basanth