Large hex values with PHP hexdec

Question

I have some large HEX values that I want to display as regular numbers, I was using hexdec() to convert to float, and I found a function on PHP.net to convert that to decima

Answer 1

1.5406319846274E+19 is a limited representation of you number. You can have a more complete one by using printf()

printf("%u\n", hexdec($h));

...will output "15406319846273792000". PHP uses floats for such big numbers, so you may lose a bit of precision. If you have to work with arbitrary precision numbers, you may try the bcmath extension. By splitting the hex into two 32-bit words (which should be safe on most systems) you should be able to get more precision. For instance:

$f = bcadd(bcmul(hexdec(substr($h, 0, -8)), 0x100000000), hexdec(substr($h, 8)));

...would set $f to 15406319846273791563.

Answer 2

As said on the hexdec manual page:

The function can now convert values that are to big for the platforms integer type, it will return the value as float instead in that case.

If you want to get some kind of big integer (not float), you'll need it stored inside a string. This might be possible using BC Math functions.

For instance, if you look in the comments of the hexdec manual page, you'll find this note

If you adapt that function a bit, to avoid a notice, you'll get:

function bchexdec($hex)
{
    $dec = 0;
    $len = strlen($hex);
    for ($i = 1; $i <= $len; $i++) {
        $dec = bcadd($dec, bcmul(strval(hexdec($hex[$i - 1])), bcpow('16', strval($len - $i))));
    }
    return $dec;
}

(This function has been copied from the note I linked to; and only a bit adapted by me)

And using it on your number:

$h = 'D5CE3E462533364B';
$f = bchexdec($h);
var_dump($f);

The output will be:

string '15406319846273791563' (length=20)

So, not the kind of big float you had ; and seems OK with what you are expecting:

Result from calc.exe = 15406319846273791563

Hope this help ;-)
And, yes, user notes on the PHP documentation are sometimes a real gold mine ;-)

Answer 3

Run into this issue while storing 64-bit keys in MySQL database. I was able to get a bit perfect conversion to a 64-bit signed integer (PHP limitation) using a few binary operators: (This code is 16x faster than bchexdec function and resulting variables are using half the memory on average).

function x64toSignedInt($k){
    $left = hexdec(substr($k,0,8));
    $right = hexdec(substr($k,8,8));
    return (int) ($left << 32) | $right;
}

MySQL signed BIGINT datatype is a great match for this as an index or storage in general. HEX(column) is a simple way to convert it back to HEX within the SQL query for use elsewhere.

Answer 4

hexdec() switches from int to float when the result is too large to be represented as an int. If you want arbitrarily long values, you're probably going to have to roll your own conversion function to change the hex string to a GMP integer.

function gmp_hexdec($n) {
  $gmp = gmp_init(0);
  $mult = gmp_init(1);
  for ($i=strlen($n)-1;$i>=0;$i--,$mult=gmp_mul($mult, 16)) {
    $gmp = gmp_add($gmp, gmp_mul($mult, hexdec($n[$i])));
  }
  return $gmp;
}

print gmp_strval(gmp_hexdec("D5CE3E462533364B"));

Output: 15406319846273791563

Answer 5

Convert HEX to DEC is easy.. But, reconstruct back hexadecimal number is very hard. Try to use base_convert ..

$hexadecimal = base_convert(2826896153644826, 10, 16);
// result: a0b0c0d0e0f1a

Answer 6

Doesn't intval(var, base) take care of it?

From the PHP Manual.