Calculate percentage increase between two decimal numbers

Question

I have a number of decimal numbers stored in a database and need to calculate the percentage increase(or decrease) between two of the numbers using PHP.

An example of two of the numbers are: 111.0516 and 111.9052 which would be an increase of 0.7628%

I found the following code somewhere. Unfortunately it doesn't seem to work with decimal numbers and rounds them up:

$num_amount = 111.0516;
$num_total = 111.9052;

function percent($num_amount, $num_total) {
$count1 = $num_amount / $num_total;
$count2 = $count1 * 100;
$count = number_format($count2, 0);
echo $count;
}

percent($num_amount, $num_total);

Ideally I need to calculate the percentage to two decimal places giving an answer of 0.77%.

Is this possible with PHP? I'm stumped. Neither my PHP or maths skills are good enough to figure it out.

Answer 1

Let's do a bit of maths.

If you have 4 euros and i give ou 2 euros, you have 6 euros, the increase is 2 / 6.

If you have x euros and I give you delta euros, you have x + delta = y euros

We can write

percentage_increase := (delta / y) * 100
                    := ((y - x) / y) * 100
                    := (1 - (x / y)) * 100

So your function becomes:

function percent($num_amount, $num_total) {
    echo number_format((1 - $num_amount / $num_total) * 100, 2); // yields 0.76
}

Codepad example

Answer 2

Just write

$count = number_format($count2, 2);

instead of

$count = number_format($count2, 0);

While keeping the rest of your code the same.