ZEND PHP 5 Certification STUDY GUIDE

XML and Web Services ” 187
• Document Type Declaration: A set of instructions that describes the accepted
structure and content of an XML file. Like entities, DTDs can either be externally
defined or embedded.
• Well-formed: An XML document is considered well-formed when it contains a
single root level element, all tags are opened and closed properly and all entities
(, &, ’, ") are escaped properly. Specifically, it must conform to all
“well-formedness” constraints as defined by the W3C XML recommendation.
• Valid: An XML document is valid when it is both well-formed and obeys a
referenced DTD. An XML document can be well-formed and not valid, but it
can never be valid and not well-formed.
A well-formed XML document can be as simple as:
Hello, World!
This example conforms fully to the definition described earlier: it has at least one
element, and that element is delimited by start and end tags. However, it is not valid,
because it doesn’t reference a DTD. Here is an example of a valid version of the same
document:
Hello, World!
In this case, an external DTD is loaded from local storage, but the declarations may
also be listed locally:
Licensed to 482634 - Amber Barrow (itsadmin@deakin.edu.au)
Hello, World!
In practice, most XML documents you work with will not contain a DTD—and, therefore,
will not be valid. In fact, the DTD is not a requirement except to validate the