For those that either haven't worked much with PHP 5.3 in their applications (or just haven't gotten around to using the feature) Rob Allen has put together an introduction to namespaces to guide you through some first steps and share some example usage.
I know that there are a lot of posts now about namespaces in PHP 5.3. This is mine which is how I learnt how they work. [...] That is, namespaces allow us to: combine libraries with the same classnames, avoid very long classnames and organise our code easily. Note that namespaces do not just affect classes. They also affect functions and constants.
He starts with the basic namespace definition (using the "namespace" keyword), shows how to import another namespace with "use" and the use of the __NAMESPACE__ constant to determine what namespace you're operating in. More information on namespaces can be found in the PHP manual.