$a = '35'; $b = '-34.99'; echo ($a + $b);
Results in 0.009999999999998
What is up with that? I wondered why my program kept reporting odd results.
Why doesn't PHP return the expected 0.01?
Because floating point arithmetic != real number arithmetic. An illustration of the difference due to imprecision is, for some floats
(a+b)-b != a. This applies to any language using floats. See an example in Python.
Since floating point are binary numbers with finite precision, there's a finite amount of representable numbers, which leads accuracy problems and surprises like this. Here's another interesting read: What Every Computer Scientist Should Know About Floating-Point Arithmetic.
Back to your problem, basically there is no way to accurately represent 34.99 or 0.01 in binary (just like in decimal, 1/3 = 0.3333...), so approximations are used instead. To get around the problem, you can:
round($result, 2)on the result to round it to 2 decimal places.
Use integers. If that's currency, store them as 3500 and 3499. Then divide the result by 100.