How to create a value reference in xml

I wish to create an xml file where one element value is depends on other value. for example:

<Server>serverName</Server>
<Port>8080</Po开发者_如何学JAVArt>
<Address>http://[value of Server]:[value of Port]/lib

i guess it can be achived with schema or dtd but i just couldnt find how to do that....

---

XML doesn't support this sort of thing in general. There's just no way to do it.