A Plugin isn't the same as what you are thinking. The terminilogy betwen Symfony and ZF are completely different.
A Plugin does not mean "Plug me in" I will work on your website. It is basically the name given to something which the developer uses in his applications which are called every page load. Rather then creating a base controller, or some other kind of helper, a plugin is used to kind of create a new abstraction from your controllers. When you create a plugin, your reason should be that the code used therein needs to be used on each page load.
On my site here are examples of some plugins
- My ACL
- My template manager. (I built a unique template engine)
Some people might even use it for caching.. however, to answer your question, you would not use plugins in the way you want.
In answer to your comment below:
No, a module is just a way of creating different layer of features on your site, which is seperate from the default module.
ie..
Example Modules
- a Blog
- a Fully fledged online store
- an administration suite
These naturally want to be seperate from your default module..
If I were you, I would implement a plugin. However you need to create your plugin yourself. Readily available ones aren't really.. provided anywhere. Fortunately however, the documentation shows you exactly how to implement something.
I would definately recommend using ZF, it is the best thing I have ever experienced in my coding career. I love it.