PHP_ROUND_HALF_UP Rounds num away from zero when it is half way there, making 1.5 into 2 and -1.5 into -2.
PHP_ROUND_HALF_DOWN Rounds num towards zero when it is half way there, making 1.5 into 1 and -1.5 into -1.
PHP_ROUND_HALF_EVEN Rounds num towards the nearest even value when it is half way there, making both 1.5 and 2.5 into 2.
PHP_ROUND_HALF_ODD Rounds num towards the nearest odd value when it is half way there, making 1.5 into 1 and 2.5 into 3.