The php BC_ functions has another option: scale.
$num1 = 10.810000000000000497;
$num2 = 2.6899999999999999467;
$result = bcadd($num1, $num2, 2);
echo $result; // 13.50
If you want to use it several times you may use bcscale() and such you won't need to specify the third option each time you use it:
bcscale(2);
$num1 = 10.810000000000000497;
$num2 = 2.6899999999999999467;
$result = bcadd($num1, $num2);
echo $result; // 13.50