This function can be used to distinguish between the two instances in which the function doc_freeform returns 1 (True). If the document identified by doc is an XML instance with a DTD declaration, but the DTD cannot be found, doc_dtd_not_found returns 1 (True).

If doc_freeform returns 0, doc_dtd_not_found will also return 0.

The doc argument specifies the identifier of the document to query. If omitted or 0, the current document is used.