math formula... not my strong suit :/

[duckduckgoose]

I need a little help writing a math formula in php.

I want to find the difference between the current number and the next number that will divide evenly into X.

For example:

I have 7 rows of data in my table.
If I divide 7 by 3, I have a remainder.
The next number that can be divided by 3 with no remainder is 9.
I'd like to return 2.

Help?

[bogu]

$rows = 7;
$divid = 3;
$tmp = $divid-($rows%$divid);

I think this should do ...