PHP RFC: More precise float value handling

• Version: 0.9
• Date: 2015-07-30
• Author: Yasuo Ohgaki yohgaki@ohgaki.net
• Status: Under Discussion
• First Published at: http://wiki.php.net/rfc/precise_float_value

This RFC is based on float like JSON numeric discussion and proposes more precise float value handling overall.

JSON is used to exchange data between systems. Although JSON RFC "6 Numbers" does not require specific implementation for float/int type, float value should be able to handle precise float values as much as possible by default.

Currently, json_encode() uses PG(precision) which is 14. IEEE 754 double supports more precision and serialize()/var_export() uses PG(serialize_precision)=17 to be more precise. Since json_encode() uses PG(precision), json_encode() removes lower digits of fraction parts and destroys original value even if PHP's float could hold more precise float value.

<?php
$j = '{ "v": 0.1234567890123456789 }';
var_dump(json_decode($j));
var_dump(json_encode(json_decode($j)));
ini_set('precision', 20);
var_dump(json_decode($j));
var_dump(json_encode(json_decode($j)));
var_dump(0.1234567890123456789);
?>

object(stdClass)#1 (1) {
  ["v"]=>
  float(0.12345678901235)
}
string(22) "{"v":0.12345678901235}"
object(stdClass)#1 (1) {
  ["v"]=>
  float(0.12345678901234567737)
}
string(28) "{"v":0.12345678901234567737}"
float(0.12345678901234567737)

PHP's float type stores “raw” IEEE 754 double and could store accurate fraction value up 17 digits.

Current PHP outputs meaningless values for oversized PG(precision)/PG(serialize_precision).

<?php
$v = 0.12345678901234567890;
var_dump($v);
ini_set('precision', 100);
var_dump($v);
?>

float(0.12345678901235)
float(0.12345678901234567736988623209981597028672695159912109375)

• PHP 7.0: Introduce PG(precision)=-1 and PG(serialize_precision)=-1 that uses zend_dtoa()'s 0 mode that rounds to nearest value. (-1 is used to indicate 0 mode)

Since JSON is used extensively for web apps, we may have JSON specific INI setting for better compatibility and ease of use.

• PHP 7.0: Add JSON_G(json_precision) - Add JSON module specific precision INI setting.

PHP 5.6's json_encode() uses PG(precision) and could lose floating point number information.

• PHP 5.6: Make json_encode() uses PG(serialize_precision).

PHP 7.0 will have 0 mode as the default. var_export()/serialize() may have different number.

FYI: Existing tests passes even when 0 mode is used.

None when old INI value is used.

PHP 7.0.0 - PG(serialize)/PG(serialize_precision) 0 mode and JSON_G(json_precision) PHP 5.6.x - JSON uses PG(serialize_precision) for float precision.

None.

PHP overall

• 0 mode (PG(precision)= -1) float outputs values rounded to nearest.

Standard module

• serialize(), var_export() - PHP7.0: Use PG(serialieze_precision) and 0 mode.

JSON

• json_encode() - PHP7.0: Use PG(serialize_precision) and 0 mode. PHP5.6: Use PG(serialize_precision)

WDDX

• wddx_serialize_vars/value() - PHP7.0: Use PG(serialize_precision) and 0 mode. It uses PG(precision) currently.

XML_RPC

• xmlrpc_encode() - PHP7.0: Use PG(serialize_precision) and 0 mode. It uses PG(precision) currently.

Not affected.

None.

precision

• hardcoded default values : 14 Unmodified
• php.ini-development values : 14 Unmodified
• php.ini-production values : 14 Unmodified

serialize_precision(PHP5.6)

• hardcoded default values : 17 Unmodified
• php.ini-development values : 17 Unmodified
• php.ini-production values : 17 Unmodified

serialize_precision(PHP7)

• hardcoded default values : -1
• php.ini-development values : -1
• php.ini-production values : -1

json_precision(NEW - PHP7 only)

• hardcoded default values : -1
• php.ini-development values : -1
• php.ini-production values : -1

None.

PHP uses “raw” IEEE 754 value internally regardless of precision settings. Therefore, this RFC does not affect internal computation.

• Remove PG(serialize_precision) setting. If we are going to remove PG(serialize_precision) in the future, we may be better to have JSON_G(json_precision) as one may need specific precision for JSON data.

Requires a 50%+1 majority (see voting)

.

.

• https://github.com/php/php-src/pull/1455

After the project is implemented, this section should contain

1. the version(s) it was merged to
2. a link to the git commit(s)
3. a link to the PHP manual entry for the feature

https://marc.info/?l=php-internals&m=143786181229714&w=2

Keep this updated with features that were discussed on the mail lists.