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Bug #68111 json_decode introduces floating point precision error
Submitted: 2014-09-27 05:00 UTC Modified: 2014-09-29 17:25 UTC
From: ryan at ryanparman dot com Assigned:
Status: Not a bug Package: JSON related
PHP Version: 5.5.17 OS: OS X
Private report: No CVE-ID: None
View Developer Edit
 [2014-09-27 05:00 UTC] ryan at ryanparman dot com 
Description:
------------
When decoding a JSON string with json_decode(), a floating point precision error gets introduced.

Issue identified in PHP 5.5.17, and verified that it does NOT occur in 5.4.33.

Test script:
---------------
$data = json_decode('{"value": 4565.85}');
echo $data->value;

Expected result:
----------------
I expected `$data->value` to have a value of `4565.85`.

Actual result:
--------------
The actual value of `$data->value` is `4565.8500000000004`.

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 [2014-09-27 05:05 UTC] ryan at ryanparman dot com
-Package: Scripting Engine problem +Package: JSON related
 [2014-09-27 05:05 UTC] ryan at ryanparman dot com 
This should be filed under JSON-related.
 [2014-09-29 16:09 UTC] aharvey@php.net
-Status: Open +Status: Not a bug
 [2014-09-29 16:09 UTC] aharvey@php.net 
Floating point values have a limited precision. Hence a value might 
not have the same string representation after any processing. That also
includes writing a floating point value in your script and directly 
printing it without any mathematical operations.

If you would like to know more about "floats" and what IEEE
754 is, read this:
http://www.floating-point-gui.de/

Thank you for your interest in PHP.

It's going to be the same value on both, just with different precision settings controlling the output.
 [2014-09-29 17:25 UTC] ryan at ryanparman dot com 
So, I would like to better understand why two versions of PHP give me different results.

----

$ php --version
PHP 5.5.17 (cli) (built: Sep 19 2014 00:50:57) 
Copyright (c) 1997-2014 The PHP Group
Zend Engine v2.5.0, Copyright (c) 1998-2014 Zend Technologies
    with Xdebug v2.2.5, Copyright (c) 2002-2014, by Derick Rethans
$ php -i | grep -i precision
precision => 14 => 14
serialize_precision => 17 => 17

----

$ php --version
PHP 5.4.33 (cli) (built: Sep 20 2014 16:20:03) 
Copyright (c) 1997-2014 The PHP Group
Zend Engine v2.4.0, Copyright (c) 1998-2014 Zend Technologies
    with Xdebug v2.2.5, Copyright (c) 2002-2014, by Derick Rethans
$ php -i | grep -i precision
precision => 14 => 14
serialize_precision => 100 => 100

----

Could it be because the `serialize_precision` value is different?
 [2014-11-19 22:22 UTC] admin at bittylicious dot com 
I've created a partially-related feature request for this to have an option for json_decode to handle all numbers as strings. If you're never going to use the result as a double internally, this can be useful.

https://bugs.php.net/bug.php?id=68456
 
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