Which value is nearer than these?

[Feudalism]

I have a value that defines the quantity of bottles of wine each box has. For eg: Each box has 50.

My customer sent me a quantity of bottles that he wants to buy, that may accord or not to the exact number of bottles per box. I can't sell an incomplete box, so all the quantity's I can sell are: 50 and it's multipliers, such as 100, 150, 200, 250.

In case the bottles of the customer doesn't fix an exact-box-quantity, I need a script that chooses from the nearer higher value. For example:

If he wants to buy 60, he has to take 100. If he wants to have 430, he needs to take 450.

Thanx!

[wonshikee]

$qty = ceil($howmanyuserwants / $howmanyperbox) * $howmanyperbox;

// eg. 60/50 = 2 * 50 = 100.