PHP: Ceil function rounding issue [duplicate]

Question

In one of my wordpress theme function, this code is being used to calculate total billable amount: ceil(($cost * (1 + ($charges / 100))) * 100);

There is a little miscalculation happening for below scenario.

Scenario:

$charges = 9;
$cost = 100;

echo ceil(($cost * (1 + ($charges / 100))) * 100);

The above code outputs 10901 whereas it should be 10900.

It works fine for other scenarios like:

$charges = 4;
$cost = 90.7;

echo ceil(($cost * (1 + ($charges / 100))) * 100); 
//outputs 9433, which is fine because manual calculation results 9432.8

Question:

1. Why is that happening?
2. How can I prevent that?
3. Any alternate function to round to next nearest integer? (only if amount is floating value)

Answer 1

The problem is that you are applying ceil to the outer expression. Try to rewrite it as:

$charges = 9;
$cost = 100;

echo ($cost + ceil($cost * $charges / 100)) * 100;

This outputs 10900 as expected.

UPDATE

As @cars10m suggested, simplifying the expression does help:

echo ceil($cost * 100  + $cost * $charges);

UPDATE 2

You can also use BCMath library to do precise math:

bcscale(6);
echo ceil(bcadd(bcmul($cost, 100), bcmul($cost, $charges)));