Description:
------------
I tested this code with different versions of php 5.6, 7.1 and 7.2 and it's the same with all.

PHP 7.2.13 (cli) (built: Dec  5 2018 07:19:53) ( NTS MSVC15 (Visual C++ 2017) x64 ) 
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.2.0, Copyright (c) 1998-2018 Zend Technologies

PHP 7.2.6 (cli) (built: May 23 2018 20:29:37) ( NTS MSVC15 (Visual C++ 2017) x64 ) 
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.2.0, Copyright (c) 1998-2018 Zend Technologies
    with Xdebug v2.6.0, Copyright (c) 2002-2018, by Derick Rethans

PHP 5.6.36 (cli) (built: Apr 25 2018 16:45:32)  
Copyright (c) 1997-2016 The PHP Group
Zend Engine v2.6.0, Copyright (c) 1998-2016 Zend Technologies

PHP 7.1.18 (cli) (built: May 24 2018 17:54:03) ( NTS MSVC14 (Visual C++ 2015) x64 ) 
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.1.0, Copyright (c) 1998-2018 Zend Technologies
    with Xdebug v2.6.0, Copyright (c) 2002-2018, by Derick Rethans


Test script:
---------------
function printBytes($bytes) {
  foreach (str_split($bytes) as $b)
    printf('%02x ', ord($b));
  echo "\n";
}

printBytes(pack('d', (double)NAN));
printBytes(pack('f', (float)NAN));


Expected result:
----------------
00 00 00 00 00 00 f8 7f 
00 00 c0 7f 


Actual result:
--------------
00 00 00 00 00 00 f8 ff  
00 00 c0 ff