How do I safely perform money related calculations in PHP?

If you're working with reasonable amounts (to a "normal person"), using floating point is not likely to be a problem, especially if you're just adding and subtracting amounts, rather than doing, say, interest calculations.

If you're looking for a quick fix, switching to integer is not likely to help you; you still have to deal with overflow. (Someone edited here to mention that if PHP encounters a number beyond the bounds of the integer type, it will be interpreted as a float instead. But then you're back to your original problem of using floating point!) Arbitrary length integers (GMP, referenced from that page) can help.)

Basically, doing this in a half-assed way works in most cases, and is cheap; doing it correctly is a serious pain. My suggestion is to look at this as a business problem: if someone complains about missing a cent, give them a dollar, and don't bother starting on a real solution until you're going to save more dollars by doing that.