How to handle xml strings in Customer Portal code?

Summary:

I am creating a widget that checks the XML of an article if a certain field from the content-schema is filled and what it's value is. I do however have problems dealing with the content-xml.

Normally in PHP you can use the SimpleXML class to handle a XML-field, but if I try to use this class I get errors stating it is not part of the class I am working from.

Can anyone tell me how to solve this?

Content (required):

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):