Possible Duplicate:
PHP Math Precision
The following php code outputs 7 but I expect 8. Why the difference?
<?php echo (int)((0.1+0.7)*10); ?>
Because due to inaccurate floating point representations, 0.1+0.7 is not exactly equal to 0.8. It might be some very tiny bit less than that. And when you use int(..), it truncates it to 7.
Others already pointed out the problem. If you're working with a fixed number of decimal places (for example, when working with money), you're better off calculating and storing cents and convertig them back to Dollars/Euros/Whatever when showing the values to the user.