php floor precision incorrect

Question

I want chop my float number, I found php doc . First I want my value always division 10000, so my function will do it.

function floorDec($value = 0, $decimals = 2) {
        $c = pow(10, $decimals);

        $a1 = $value / 10000;

        printf("a1: %lf, type: %s\n", $a1, gettype($a1));

        $a2 = $a1 * $c;

        printf("a2: %lf, type: %s\n", $a2, gettype($a2));

        $a3 = floor($a2);

        printf("a3: %lf, type: %s\n", $a3, gettype($a3));

        $a4 = $a3 / $c;

        printf("a4: %lf, type: %s\n", $a4, gettype($a4));

        return $a4;
}

$a =  96684700;

var_dump(floorDec($a, 2));

Result

a1: 9668.470000, type: double
a2: 966847.000000, type: double
a3: 966846.000000, type: double
a4: 9668.460000, type: double
float(9668.46)

The variable $a3 is not I want. I have no idea why. Thanks your help.

Answer 1

PHP is behaving correctly. This is just a side effect of the way floating point numbers behave.
They are sometimes just a little bit inaccurate:

printf("%.99f", $a2);

prints

966846.99999999988358467817306518554687500000000000000000000

so of course if you floor() that it'll be 966846.

Perhaps (int)round($a2); does what you want:

966847.00000000000000000000000000000000000000000000000000000

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If you need absolute precision up to a certain decimal point, have a look at these functions: https://secure.php.net/manual/en/ref.bc.php