Is Codeigniter 3 php 7 ready?

CI 3 is based on php standards for PHP 5.3.X.
CI team has made it to work on servers with all PHP versions 5.3 +
The same thing is made and for PHP 7 so it wont matter in which version is your server working.

This is only about the SYSTEM folder which is the framework Core.
What is written in the APP is Developer work and only you / him may say which PHP version it supports.

If you have used something from PHP 5.6 at your app it wont work on PHP 5.5 or less.
Same for PHP 7

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