Can I use a Raspberry Pi as a web server for a CodeIgniter app? I can't find anything online. I saw tutorials on installing Wordpress but no CodeIgniter...
This is really a question about an operating system, not a brand of device, although the conflation/confusion is a common one. Point being, the answer depends mostly on what operating system you want to use.
The most widely used operating system on the Pi platform is Raspbian, a GNU/Linux distribution. You will find lots of references online to the "LAMP" stack, which refers to a common MVC implementation where LAM is for "linux apache mysql" and the p is alternately for python, perl, or PHP. If you can use CodeIgniter with the Apache server, you should be able to use it on linux therefore on a Raspbian powered Pi. There are other choices of web server as well that should work in this scenario.
At a glance I think CodeIgniter is written entirely in PHP, so you should not have to compile anything from C unless it has dependent modules requiring such that aren't available directly through the Raspbian repos, in which case you'll have to build them, which is not impossible.
To check for modules available directly through Raspbian use
apt search [module name] | grep "^php", e.g.,
apt search xml | grep "^php"
I believe PHP has its own package manager,
pecl, which can also be used and should take care of building things if that is necessary. Always rely on the distro package manager (
apt) first; sometimes you may need a more recent version of something, which
pecl should be able to get. It is fine to install a package with both in this case, meaning, you don't have to remove the one from
apt if you install it then realize you need one from
pecl, because the latter should take precedence.
If you are looking for more information, don't include "Raspberry Pi" in your search terms when something more general is more appropriate (such as "linux" or "apache").