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

Taken from that same page: "Note: CodeIgniter will run on PHP 5. It simply does not take advantage of any native features that are only available in that version." So, it does run on PHP5, just doesn't use PHP5 only features for the sake of backwards compatibility. For PHP5 one can use KohanaPHP, as mentioned by another comment :)