php code:
$am = $_POST['amount'];
$ret = ceil($am/0.05);
EDIT:
NB: ceil rounds up to nearest integer value, which I assumed from your post. However, if you needed to round up such that you end up to the nearest pence/cent, you'd have to use round() and an appropriate precision value, along with adding 0.005 to ensure you end up with the correct rounded up value...
php code:
$am = $_POST['amount'];
$ret = round(($am/0.05)+0.005, 2);
EDIT 2:
Thinking about it, dividing an integer with no more than 2 dp by 0.05 (effectively multiplying by 20) will mean there will be at most 1dp, so that last edit can be ignored, but kept for reference
/ 
