## Chapter Summary

4. A DOMDocument object consists of nodes that are represented by objects of the DOMNode class or one of its derived classes. The most important derived classes are DOMElement and DOMText
5. A DTD is a sequence of rules that describe the legal child elements and attributes for each element type. An XML document can contain its DTD or refer to a DTD that is stored elsewhere. When referencing an external DTD, you must supply a file name or URL
6. It is best to use DTDs with XML files. When using DTDs, you should tell the parser to validate the document and to ignore white space. This greatly simplifies the implementation of the parser functions

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