I can't seem to find a reference for CodeIgniter's naming convention for interfaces. Is there one? Can anyone suggest one?
FYI: CodeIgniter uses this for classes: Foo_bar. Zend uses something like this for interfaces: Foo_Bar_PluginInterface.
Not sure exactly what you mean by interface, but if you mean Class names, then after a quick look at their documentation tells me you are right with Foo_bar.
Update
I do not think there is a set convention for the use of interfaces in Codeigniter, at least not stated by Ellislab.
There isn't a naming convention for Codeigniter nor for any other EllisLab product. ExpressionEngine their flagship commercial content management system doesn't have a single interface insight.
Considering Codeigniter hasn't long dropped support for PHP4, it might explain why interfaces aren't used. May I ask why you want to use interfaces in Codeigniter anyway? I can't see any reason too, but yet again, my use of interfaces and abstract classes in PHP is infrequent.