In the docs of CodeIgniter it states,
PHP version 5.4 or newer is recommended.
I looked on forums etc. and wasn't able to find a definitive yes/no on this, even though the above sentence implies a 'yes', I wasn't sure if that extended to the (now new) PHP7.
I have tried a simple CI3 example with PHP7, and all I can report is so good, so far.
Yes. The latest v3.0.4 changelog explicitly has changes to use PHP7 functionality where it can e.g.
Updated Security Library method get_random_bytes() to use PHP 7’s random_bytes() function when possible.
Update August 2018 There's also items in the change log for 3.1.8 which relate to fixes for PHP 7.2, as well as 3.1.7 which specifically mention fixes for PHP 7