Calculations

Hi there list. Thanks for your help with my other questions. I was carrying on working through, and stumbled across a problem when trying to times something by the 'power' of something. By this I mean..in maths you can write 3 * (2 ^ 3), the answer to which would be 24. (as 2 ^ 3 = 8). The reason that I need this, is because I'm trying to double a variable x amount of times (x is set by another variable). For example my code is: $cost = $farm_cost * (2 ^ $farm_level); Any ideas how I would go about changing my calculation, as php dosn't seem to recognise ^ meaning 'to the power of'. Regards, Alex.