For building a web application you spend a lot of time in writing the same code again and again. Frameworks provide you a starting block and minimize the amount of code needed to build a website.
CodeIgniter is PHP driven framework but it's not a PHP substitute. Diving into CodeIgniter doesn?t mean you are leaving PHP behind. PHP is a server-side scripting language for building dynamic web-based applications.
A person using CodeIgniter must be familiar with PHP. You need to have a good knowledge about PHP like its basic syntax and how it interacts with database and HTML.