Well, we have pretty much let the cat out of the bag already.
We have shown several examples of entity references above.
In short, Entity References refer to the key that unlocks an
entity which has been declared in an Entity Declaration.
Entity References follow the simple syntax of:
&ENTITY_NAME;
such as
&letterhead;
WARNINGS:
Entities MUST be declared in an XML document before they are
referenced.
Note that there may not be any whitespace embedded in an
entity reference. In other words, & letterhead; or &letterhead ;
will cause errors.
Though entities may refer to other entities, they may not be
self-referential. So they may not reference other entities that
reference them in return.
References to entities may not appear in the DOCTYPE
declaration.
The text that the entity references must be well-formed XML.
As you might expect parameter entity references work much like
general entity references. In this case, we use a "%" sign
instead of a "&".
%PARAMETER_ENTITY_NAME;
Now, you have already seen that entity references can take the
place of regular character data and you have seen how useful
that is. Before we leave the subject, I would only mention that
you could also use entity references within tag attributes.
For example, consider the following:
