According to the official documentation, PHP namespaces have been designed to prevent name collisions between classes from different packages and to avoid the use of very long names in the code to refer to classes or functions—nobody really wants to have to deal with something called Zend_Db_Adapter_Mysqli or PHPUnit_Framework_Constraint_IsInstanceOf, after all. This means that namespaces help a developer write code that is both more concise and clearer—a direction which is always an improvement towards expressiveness.
Within the PHP implementation of namespaces, these names will be ideally refactored to Zend\Db\Adapter\Mysqli and PHPUnit\Framework\Constraint\IsInstanceOf, where \ is the namespace separator. In the codebase, however, there will typically be very few references to these classes with their fully qualified name, because it is possible to import entire namespaces in a script and then use the class names directly, making the code easier to follow and unambiguous to write.
In fact, the definition of a namespace class itself does not contain its fully qualified name. For example, this would be the source file of an hypothetical MyLibrary\TypeOfComponents\MyClass class:
The convention when writing namespace-enabled code is that of creating a folder structure that reflects the individual components of a namespace (for example, MyClass would be in the MyLibrary/TypeOfComponents directory. This helps standardizing the autoloading process.
Read the full story PHP 5.3 namespaces for the rest of us | php|architect.