This function returns the generic identifier name from the DOCTYPE declaration for the document given by doc. If doc is omitted or 0, the current document is used.

Note that the return value is normally the root element declared in the DTD (for example, “book”), but it may be a different element name if the file had a DOCTYPE line specifying a lower level element (for example, “chapter”). This function is different from the doc_type function that returns the external name of the document type which need not be an element name.