In Zend FW 2.0 the standard src folder (and namespace) structure is the following:

src\MyNamespace\Model - models
src\MyNamespace\Controller - controllers
src\MyNamespace\Helper - helpers, utilities

However I wanted to follow the same logic for interfaces I failed. The following does not work:
src\MyNamespace\Interface - interfaces

That’s because “Interface” (and interface) is a reserved keyword in PHP.

Mindnote here for next time –  I have to make exception for interfaces and name my namespace (folder) for interfaces in plural:
src\MyNamespace\Interfaces