Well I found a number of sites that present one to the public; but, as you have probably noticed, the aren't giving away the backend details (or are they ... if you can find one in Javascript, (browser/View Source) and don't care much for integrity when it comes to intellectual property, you can have one right away....)
OTOH, if you know how to make the calculations, you might be able to write it down in "pseudo code" --- you never know, someone might be willing to PHPize it for ya.....