No, don't even think of learning CodeIgniter. Besides being pretty much dead (version 3 has been coming out for, what, about 2 years now?), it has terrible architecture. The superglobal singleton is just awful and will kill you slowly.
This. I don't understand why CodeIgniter gets mentioned in the same breath as Zend or Symphony, or any other fully-fledged framework. CI is, to start with, much smaller: it's tiny, the sort of thing you could knock out in a week. It also has deep-rooted architectural issues that are preventing some of the longest standing bugs being fixed (for example, take a look at the router). Lastly it's not anywhere near as modular as it purports to be, and the code and architecture quality is rather poor.
The only reason why I mentioned it is because it's extremely easy - for a beginner, it's not a bad choice because it doesn't overwhelm you. Take baby steps.
Just because something it easy does not make it the right choice.
Conversely, it is a bad choice because it teaches you bad practices and you will be forced to implement so many hacks on any reasonable sized project. Just don't use it.
I found CodeIgniter to be really useful for a self-taught, spaghetti-style PHP programmer. It is the only framework that most such programmers can grasp by themselves using tutorials and docs available online. Any other framework, and they would require a lot of hand-holding.
Switching from CI to something better later is really not that hard and it's much, much easier that learning one of the modern frameworks from scratch.
I'm talking about self-taught begginers without CS degree or working-in-a-large-team experience.
But then you're better off with something like Silex. It's tiny and easy to pick up, but you're using a solid software and along the way get familiar with Symfony components as well.
