Description:
------------
We have a hex string like this: "eb3dd796efda2409" received from an upstream system. This is a big-nibble hex string containing this integer value: -1495802457948019703
This is a 64-bit signed integer.

When converting this using hexdec this is converted to a float and returns the wrong value, when assigned to a variable as a hex string and converted using intval() it loses precision.

When looking at the documentation: http://php.net/manual/en/function.hexdec.php it mentions this:
> The function can convert numbers that are too large to fit into the platforms integer type, larger values are returned as float in that case.

However, this happens on a 64-bit compiled PHP where this integer should fit into the platform's integer size.
php --version
PHP 7.2.3-1 (cli) (built: Mar  6 2018 11:15:04) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.2.0, Copyright (c) 1998-2018 Zend Technologies
    with Zend OPcache v7.2.3-1, Copyright (c) 1999-2018, by Zend Technologies
php -a
Interactive mode enabled

php > echo PHP_INT_MAX;
9223372036854775807
php > echo PHP_INT_SIZE;
8

As a side note:
We ended up using pack/unpack to avoid the interim float conversion, and we also had to use "J" as the unpack conversion type to get the right value which implies big-endianness in the packed value. I am not sure this is correct either as doing this conversion in other languages works just fine without specifying endianness. (tried in Python and Go)
(I can file a separate bug if this is also not intended)

Test script:
---------------
$strhex = "eb3dd796efda2409";
$hex = 0xeb3dd796efda2409;

echo hexdec($strhex);
echo $hex;
echo intval($hex);

var_dump(unpack('J', pack("H*", $strhex)));


Expected result:
----------------
-1495802457948019703
-1495802457948019703
-1495802457948019703
array(1) {
  [1]=>
  int(-1495802457948019703)
}

Actual result:
--------------
1.6950941615762E+19
1.6950941615762E+19
-1495802457948020736
array(1) {
  [1]=>
  int(-1495802457948019703)
}