Best way to allow plugins for a PHP application

I am starting a new web application in PHP and this time around I want to create something that people can extend by using a plugin interface. How does one go about writing 'hooks' into their code so that plugins can attach to specific events?

Achievements / Badges system

I have been browsing this site for the answer but I'm still a little unsure how to plan a similar system in its database structure and implementation.

What are the alternatives to MVC architecture for web development?

I have searched around for alternatives just to see what is available, but I can hardly find anything.

Call graph of the whole application

Is there a non-toyish tool that can create a call graph of the whole application? I don't mean just getting a picture or drawing call graph by means of pointing method-by-method.

iPhone Compiler Fails: No architectures to compile for

Hey guys, I'm trying to implement this in my iPhone app: http://code.google.com/p/core-plot/downloads/detail?name=alpharelease_0.1.zip&can=2&q=&sort=-uploaded . I figured I'd first try to run the sample they provide. I'm attempting to open and compile the project located in /Source/examples/CPTestApp-iPhone/ . It says my base SDK was missing right off of the bat, so I edited the Project settings and the Active Target to use the iOS4 SDK, which I've done before for samples and had work. I'm not sure what to do. I'm running one of the newest Unibody Macbooks, with 10.6.4.