Easiest way to user DomDocument->getElementById()?

According to the php docs, the method DomDocument->getElementById() will not work unless the document is validated using a DTD (not schema): "According to the DOM standard this requires a DTD which defines the attribute ID to be of type ID. You need to validate your document with DOMDocument->validate() or DOMDocument->validateOnParse before using this function." I cannot use a schema in ordet to "activate" DomDocument->getElementById()? /Erik