Very Basic yet Very effective statement...
Most importantly, you need to be aware that the float data type is not always capable
of representing numbers in the way you expect it to. Consider, for example this
very simple statement:
echo (int) ((0.1 + 0.7) * 10);
You would expect that the expression ((0.1 + 0.7) * 10) would evaluate to 8 (and, in fact, if you print it out without the integer conversion, it does). However, the statement above outputs 7 instead. This happens because the result of this simple arithmetic expression is stored internally as 7.999999 instead of 8; when the value is converted to int, PHP simply truncates away the fractional part, resulting in a rather
significant error (12.5%, to be exact).
The lesson that you need to take home from all this is simple: know the limitations of your numeric data types, and plan around them. Whenever the precision of your calculation is a relevant factor to the proper functioning of your application, you should consider using a the arbitrary precision functions provided by the BCMath extension (you can search for it in your copy of the PHP manual) instead of PHP's built-in data types.