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.
CodeIgniter contains libraries, simple interface and logical structure to access these libraries, plug-ins, helpers and some other resources which solve the complex functions of PHP more easily maintaining a high performance. It simplifies the PHP code and brings out a fully interactive, dynamic website in a much shorter time. It supports PHP version of 5.2.6 or newer and MySQL version 4.1 or newer. It makes your web more robust and your code easier to read and maintain. It is a free toolkit, lightweight and easier to install.
A person using CodeIgniter must be familiar with PHP. You need to have a good knowledge of PHP like its basic syntax and how it interacts with database and HTML.