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Upgrading to PHP 5 by Adam Trachtenberg

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XML Namespaces

XML Namespaces let you place a set of XML elements inside a separate “area” to avoid tag name clashes. This is an important feature because it allows XML documents to be extended and combined. Unfortunately, using XML namespaces is tricky. For something that initially seems very straightforward, there’s a surprising amount of explanation required.

Why Use Namespaces?

Using XML Namespaces, developers can work together to define a common set of markup for different sets of data, such as RSS items, meta-information about pages on the Internet, or books. When programmers everywhere represent related information using the same set of elements in the same namespace, then everyone can create powerful applications based on a large set of shared data.

That’s the theory, anyway.

On a more practical side, avoiding tag name clashes is still an issue because it’s useful to modify XML documents. Clashes aren’t a problem when everyone is working with a fixed set of elements. However, you can run into trouble if you allow others to extend a document by adding their own elements.

For example, you may decide to use <title> to refer to the title of a web page, but your friend used <title> as the title of a person, such as Mister or Doctor. With XML Namespaces, you can keep <html:title> distinct from <person:title>.

Some languages have a similar concept, where functions and objects belonging to a package can be namespaced together. PHP does not support namespaces, which is why you may ...

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