XML in a Nutshell, 3rd Edition

External Parsed General Entities

The footer example is about at the limits of what you can comfortably fit in a DTD. In practice, web sites prefer to store repeated content like this in external files and load it into their pages using PHP, server-side includes, or some similar mechanism. XML supports this technique through external general entity references, although in this case the client, rather than the server, is responsible for integrating the different pieces of the document into a coherent whole.

An external parsed general entity reference is declared in the DTD using an ENTITY declaration. However, instead of the actual replacement text, the SYSTEM keyword and a URL to the replacement text is given. For example:

<!ENTITY footer SYSTEM "http://www.oreilly.com/boilerplate/footer.xml">

Of course, a relative URL will often be used instead. For example:

<!ENTITY footer SYSTEM "/boilerplate/footer.xml">

In either case, when the general entity reference &footer; is seen in the character data of an element, the parser may replace it with the document found at http://www.oreilly.com/boilerplate/footer.xml. References to external parsed entities are not allowed in attribute values. Most of the time this shouldn’t be too big a hassle because attribute values tend to be small enough to be easily included in internal entities.

Notice we wrote that the parser may replace the entity reference with the document at the URL, not that it must. This is an area where parsers have some leeway ...