Consider Kohana, then, which forked off from CodeIgniter years ago and learned from the many mistakes CI made (most notably being a complete inability to adapt), and was designed with PHP 5 in mind from day one.
They've rebuilt much of the base code on top of PHP 5 natively now instead of being built on PHP 4 with hacks to make it also work with 5.
Although it is still in development, it has been said that it is stable.