PHP7.1 json_encode() Float Issue

问题

This isn't a question as it is more of a be aware. I updated an application that uses json_encode() to PHP7.1.1 and I was seeing an issue with floats being changed to sometimes extend out 17 digits. According to documentation, PHP 7.1.x started to use serialize_precision instead of precision when encoding double values. I'm guessing this caused an example value of

472.185

to become

472.18500000000006

after that value went through json_encode(). Since my discovery, I have reverted back to PHP 7.0.16 and I no longer have the issue with json_encode(). I also tried to update to PHP 7.1.2 before reverting back to PHP 7.0.16.

The reasoning behind this question does stem from PHP - Floating Number Precision, however the end all reason for this is because of the change from precision to serialize_precision usage in json_encode().

If anyone does know of a solution to this problem, I'd be more than happy to listen in on the reasoning/fix.

Excerpt from multidimensional array (before):

[staticYaxisInfo] => Array
                    (
                        [17] => stdClass Object
                            (
                                [variable_id] => 17
                                [static] => 1
                                [min] => 0
                                [max] => 472.185
                                [locked_static] => 1
                            )

                    )

and after going through json_encode()...

"staticYaxisInfo":
            {
                "17":
                {
                    "variable_id": "17",
                    "static": "1",
                    "min": 0,
                    "max": 472.18500000000006,
                    "locked_static": "1"
                }
            },

回答1:

This drove me nuts for a bit until I finally found this bug which points you to this RFC which says

Currently json_encode() uses EG(precision) which is set to 14. That means that 14 digits at most are used for displaying (printing) the number. IEEE 754 double supports higher precision and serialize()/var_export() uses PG(serialize_precision) which set to 17 be default to be more precise. Since json_encode() uses EG(precision), json_encode() removes lower digits of fraction parts and destroys original value even if PHP's float could hold more precise float value.

And (emphasis mine)

This RFC proposes to introduce a new setting EG(precision)=-1 and PG(serialize_precision)=-1 that uses zend_dtoa()'s mode 0 which uses better algorigthm for rounding float numbers (-1 is used to indicate 0 mode).

In short, there's a new way to make PHP 7.1 json_encode use the new and improved precision engine. In php.ini you need to change serialize_precision to

serialize_precision = -1

You can verify it works with this command line

php -r '$price = ["price" => round("45.99", 2)]; echo json_encode($price);'

You should get

{"price":45.99}

回答2:

As a plugin developer I don't have general access to the php.ini settings of a server. So, based on Machavity's answer I wrote this small piece of code that you can use in your PHP script. Simply put it on top of the script and json_encode will keep working as usual.

if (version_compare(phpversion(), '7.1', '>=')) {
    ini_set( 'serialize_precision', -1 );
}

In some cases it is necessary to set one more variable. I am adding this as a second solution because I am not sure if the second solution works fine in all cases where the first solution has proven to work.

if (version_compare(phpversion(), '7.1', '>=')) {
    ini_set( 'precision', 17 );
    ini_set( 'serialize_precision', -1 );
}

回答3:

I had the same problem but only serialize_precision = -1 did not solve the problem. I had to do one more step, to update the value of precision from 14 to 17 (as it was set on my PHP7.0 ini file). Apparently, changing the value of that number changes the value of the computed float.

回答4:

The other solutions didn't work for me. Here's what I had to add at the beginning of my code execution:

if (version_compare(phpversion(), '7.1', '>=')) {
    ini_set( 'precision', 17 );
    ini_set( 'serialize_precision', -1 );
}

回答5:

I was encoding monetary values and had things like 330.46 encoding to 330.4600000000000363797880709171295166015625. If you don't wish to, or can't, change the PHP settings and you know the structure of the data in advance there is a very simple solution that worked for me. Simply cast it to a string (both the following do the same thing):

$data['discount'] = (string) $data['discount'];
$data['discount'] = '' . $data['discount'];

For my use case this was a quick and effective solution. Just note that this means when you decode it back from JSON it will be a string since it'll be wrapped in double quotes.

回答6:

I solved this by setting both precision and serialize_precision to the same value (10):

ini_set('precision', 10);
ini_set('serialize_precision', 10);

You can also set this in your php.ini

回答7:

As for me the problem was when JSON_NUMERIC_CHECK as second argument of json_encode () passed, which casting all numbers type to int (not only integer)

回答8:

Store it as a string with the exact precision that you need by using number_format, then json_encode it using the JSON_NUMERIC_CHECK option:

$foo = array('max' => number_format(472.185, 3, '.', ''));
print_r(json_encode($foo, JSON_NUMERIC_CHECK));

You get:

{"max": 472.185}

Note that this will get ALL numeric strings in your source object to be encoded as numbers in the resulting JSON.

回答9:

You could change the [max] => 472.185 from a float into a string ([max] => '472.185') before the json_encode(). As json is a string anyway, converting your float values to strings before json_encode() will maintain the value you desire.

来源：https://stackoverflow.com/questions/42981409/php7-1-json-encode-float-issue

标签

php

json

precision

php-7.1