money maths

I'm building a demo shopping cart and have sorted out most things but this: I have php work out subtotals by multiplying $quantity * $price ... this works fine except that 14.95 * 2 comes out as 29.9 not 29.90. In other words, how to add a zero so that there are always two decimal places. Any solution to this, including regex? I can't find anything among the maths functions. Thanks Mick