From here: http://codeigniter.com/user_guide/overview/at_a_glance.html

"CodeIgniter is written to be compatible with PHP 4. Although we would have loved to take advantage of the better object handling in PHP 5 since it would have simplified some things we had to find creative solutions for (looking your way, multiple inheritance), at the time of this writing PHP 5 is not in widespread use, which means we would be alienating most of our potential audience. Major OS vendors like RedHat are moving slowly to support PHP 5, and they are unlikely to do so in the short term, so we felt that it did not serve the best interests of the PHP community to write CodeIgniter in PHP 5.

Note: CodeIgniter will run on PHP 5. It simply does not take advantage of any native features that are only available in that version."

if you are looking into Code Igniter, i suggest that you look into http://kohanaphp.org/home - it is a fork of CodeIgniter.

CodeIgniter does support PHP5. But you will have to turn off warnings, etc.