How to round down to the nearest significant figure in php

Question

Is there any slick way to round down to the nearest significant figure in php?

So:

0->0
9->9
10->10
17->10
77->70
114->100
745->700
1200->1000

?

Answer 1

$numbers = array(1, 9, 14, 53, 112, 725, 1001, 1200);
foreach($numbers as $number) {
    printf('%d => %d'
            , $number
            , $number - $number % pow(10, floor(log10($number)))
            );
    echo "\n";
}

Unfortunately this fails horribly when $number is 0, but it does produce the expected result for positive integers. And it is a math-only solution.

Answer 2

Here's a pure math solution. This is also a more flexible solution if you ever wanted to round up or down, and not just down. And it works on 0 :)

if($num === 0) return 0;
$digits = (int)(log10($num));
$num = (pow(10, $digits)) * floor($num/(pow(10, $digits)));

You could replace floor with round or ceil. Actually, if you wanted to round to the nearest, you could simplify the third line even more.

$num = round($num, -$digits);

Answer 3

If you do want to have a mathy solution, try this:

function floorToFirst($int) {
    if (0 === $int) return 0;

    $nearest = pow(10, floor(log($int, 10)));
    return floor($int / $nearest) * $nearest;
}