SAX to DOM Implementation Notes

When a DOMDocument object is built from SAX events, the document is locked and cannot be modified until the endDocument method is called. The document can, however, be read anytime after startDocument method is called. The following provides an overview of how the document is locked as it is built.

http://doc.ddart.net/xmlsdk/htm/sax_devgd_hdi_saxtodom_884z.htm