Object oriented style:
class XMLWriter {Procedural style:
bool xmlwriter_start_dtd ( resource xmlwriter, string qualifiedName [, string publicId [, string systemId]] )Starts a DTD.
xmlwriter仅用于过程调用。resource 是被修改的 XMLWriter 资源。此资源来自对 xmlwriter_open_uri() 或 xmlwriter_open_memory() 的调用
qualifiedNameThe qualified name of the document type to create.
publicIdThe external subset public identifier.
systemIdThe external subset system identifier.