Cos goes way back to the 80's when it was called coslib and written in C. It was converted into PHP 3 somewhere in the 90's, then it turned into a class with static methods in the 2000's (PHP 4), and survives now on PHP 7 just because so much (legacy) code still depends on it. It contains various obsolete functions that are now replaced by language constructs, like Cos::ifset() that is now replaced bij the ?? operator in PHP 7.
Don't use it in new projects but still fun to use as a time machine into the 80's
composer install cosninix/cos
As Boele used to say: No need for documentation because a good developer should be able to read code.