For starter, i have been using Code Igniter for over a year and it was introduced by a friend of mine as he was explaining on how essential to have a PHP Framework to speed up your web development. I was doubt at that time, but after I tried it by myself for a couple of months, I was truly amazed on the fact that it does rapidly save your efforts and time.
Code Igniter is an open source PHP framework smilar to Zend and CakePHP if you know what i'm talking about. I would strongly recommend CI because it's lightweighted as compared to bloated Zend Framework. It introduces the idea of MVC (Model - View - Controller) approach in developing a website. For those PHP developers, you would normally put the whole HTML content, database connection and query, and PHP logic in one file with .php extension.
Now, what Code Igniter does, it separates the HTML content into "views" folder, database connection and query into "models" folder, and PHP logic into "controllers" folder. From my point of view, it gradually cleans up your code and makes it easy for web designers and developers to collaborate together, as a web designer should only need to modify the "views" folder, and there won't be any confusion in seeing PHP and/or MySQL code.