Round to the nearest X

I realize this is a dumb question but I just can't come up with an equation that will do this and I don't see an internal PHP function (for version 4.3) that will do this... Basically I need to come up with an equation/algorithm that will round any number up/down to the nearest X. I tried using y +/- (y mod x) but after I started putting in some test numbers, it didn't always work. For example, 37678 + ( 37678 % 500 ) did not round up to the nearest 500. Any ideas? thnx, Chris