I am working on a site based on Chamilo LMS.
I want to change some styles and form fields in the login form. I found the template for the login form, but I am not able to find the PHP code that handles the logic for the login form. Where is this code located?
It depends on your version, but you should find a reasonnable part of the login form logic in main/inc/local.inc.php.
If you are going to modify the login form, I would kindly suggest you look into the "custom pages" feature. Search for the custompages/ folder and, if you're in 1.9.10.2 or 1.10.0, make a copy of the files there before you rename them (to remove the "dist" part) and update them to suit your needs.