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.

Furthermore, Code Igniter does have extra useful functionalities such as "libraries" and "helpers" and believe me they are practically easy and ready to be deployed. Personally, i would give an extra credit on the validation library, as it cleans user inputs very well, validates pretty tight and secured in a sense. Additionally, it will clean your URL by default (SEO capability for better indexing at Google search engine), you just need to change a couple of settings in "config", and you are good to go.

To get a good look on how Code Igniter manages to separate its heaps of functionalities, you could see the screenshot below depicting a directory structure that you'll have soon after you've installed Code Igniter on your server (by transferring the whole framework through FTP):

For more information, you should visit the Code Igniter user documentation, it is by far the most comprehensive and complete user guide i have seen for an open source PHP framework. On further note, if you are having difficulties or questions in regard to Code Igniter framework, you could post anytime on their forum, the responses are pretty quick and reliable from my experience.