PHP5

[eluser]Michael Wales[/eluser]

Incidentally, CodeIgniter uses a different base automatically depending on whether your server uses PHP4 or PHP5, giving you PHP5’s advantages [...]

It always cracks me up when people run off to Kohana, for this reason alone. If someone's willing to jump ship to another framework, merely because of the versions of PHP that CodeIgniter supports, well good riddance. I'd rather stick to something that works beautifully, has an excellent team backing it (that actually uses it on a daily basis, with real projects, that really make money).