I've got a grid of 10 square list items. A bit like a gallery. If the user adds another item there will be 11. However this will look strange as the 11th item will be on its own in a new row. How can I use PHP to round up to the nearest 5 and add in the some blank/dummy list items?
You could use the modulo operator to identify the remainder of a division:
10 % 5 = 0
11 % 5 = 1
12 % 5 = 2
13 % 5 = 3
14 % 5 = 4
15 % 5 = 0
With that you can identify if (and how large) such an uncomplete row would be. Knowing how many elements are in that last uncomplete row oviously allows you to calculate the number of remaining cells to fill the row.
($y+(($y%$x)?($x-($y%$x)):0))
...where $y is the number of items(e.g. 11) and $x is the number of items in a row(e.g. 5)