PHP Math fail (hexadecimal)

Question

Im trying to do a php multiplication of two 32bit long hexadecimal valuey with PHP and it seems it is messing up this calculation, the same happens if i multiplicate it as decimal value.

The calculation is as example: 0xB5365D09 * 0xDEBC252C

Converting to decimal before with hexdec doesnt change anything.

The expected result should be 0x9DAA52EA21664A8C but PHPs result is 0x9DAA52EA21664800

Example:

<?php
$res = 0xB5365D09 * 0xDEBC252C;
echo dechex(intval($res));
?>

What i am doing wrong here?

PHP8.2 running on debian, 64bit AMD.

Answer 1

So, for others to find the answer: Someone stated in the comments to my question, that the result is above the PHP_MAX_INT limit. So when PHP handle it as a FLOAT, there will be some precision of the result lost. I got it to work using bcmath. In my case, i didnt do math with the result any further so i grabbed some piece of code from here, and made a simple function which does what i need. Here you can see a minimum-example:

function bcmul_hex($h1, $h2) {
    $dec = bcmul($h1, $h2);
    $hex = '';
    do {    
        $last = bcmod($dec, 16);
        $hex = dechex($last).$hex;
        $dec = bcdiv(bcsub($dec, $last), 16);
    } while($dec>0);
    return $hex;
}

echo bcmul_hex(0xB5365D09, 0xDEBC252C);

Here is a live example.