PHP json_encode() 小數精度問題

環境：PHP 7.3.12

問題：

$json_arr = ["aa" => 0.1];
echo json_encode($json_arr, JSON_UNESCAPED_UNICODE);

輸出結果

{"aa":0.10000000000000001}

浮點數 0.1 轉成 JSON，變成 0.10000000000000001 

以前沒問題的程式，突然發現有這情況。

原來 PHP 從 7.1 開始，json_encode() 改使用 serialize_precision 的設定值處理小數精度。

而我的環境從 PHP 5.6 慢慢升級到 PHP 7.3.12，設定檔還是沿用舊版的。

舊版使用 precision 設定值處理 json_encode() 的浮點數精度。EG(precision)

7.1版後使用 serialize_precision 設定值處理 json_encode() 的浮點數精度。PG(serialize_precision)

舊版 PHP，php.ini 中 precision = 14，serialize_precision = 17

; The number of significant digits displayed in floating point numbers.
; http://php.net/precision
precision = 14

; When floats & doubles are serialized store serialize_precision significant
; digits after the floating point. The default value ensures that when floats
; are decoded with unserialize, the data will remain the same.
serialize_precision = 17

7.1版後，php.ini 中 precision = 14，serialize_precision = -1

; The number of significant digits displayed in floating point numbers.
; http://php.net/precision
precision = 14

; When floats & doubles are serialized, store serialize_precision significant
; digits after the floating point. The default value ensures that when floats
; are decoded with unserialize, the data will remain the same.
; The value is also used for json_encode when encoding double values.
; If -1 is used, then dtoa mode 0 is used which automatically select the best
; precision.
serialize_precision = -1

據 https://bugs.php.net/bug.php?id=74221 上說

 [2017-03-07 23:06 UTC] nikic@php.net
 You can restore the previous behavior by setting serialize_precision to the value of precision,
 which is 14 by default.
 Alternatively and preferably, you can set serialize_precision to -1.
 In this case PHP will automatically determine the minimal precision
 necessary to still accurately represent the number.

解決方法一：將 serialize_precision 設為 14，也就是舊版 json_encode() 處理使用的 precision = 14 這個值

ini_set('serialize_precision', 14);
$json_arr = ["aa" => 0.1];
echo json_encode($json_arr, JSON_UNESCAPED_UNICODE);

輸出結果

{"aa":0.1}

解決方法二：將 serialize_precision 設為 -1，也就是新版的預設值。

ini_set('serialize_precision', -1);
$json_arr = ["aa" => 0.1];
echo json_encode($json_arr, JSON_UNESCAPED_UNICODE);

輸出結果

{"aa":0.1}

precision、serialize_precision 都可設定為 -1，官網說明是會使用較佳的算法。
https://www.php.net/manual/en/ini.core.php#ini.precision

precision int
The number of significant digits displayed in floating point numbers.
-1 means that an enhanced algorithm for rounding such numbers will be used.

https://www.php.net/manual/en/ini.core.php#ini.serialize-precision

serialize_precision int
The number of significant digits stored while serializing floating point numbers.
-1 means that an enhanced algorithm for rounding such numbers will be used.

https://wiki.php.net/rfc/precise_float_value

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).
The RFC also proposes changing ini for JSON precision to PG(serialize_precision).

結論：

設成新版預設值 serialize_precision = -1，應該是最佳的解法。

如果不放心，怕又遇到非預期情況，且可接受使用字串形式，也可先將浮點數轉成字串再處理。

$json_arr = ["aa" => (string) 0.1];
echo json_encode($json_arr, JSON_UNESCAPED_UNICODE);

輸出結果

{"aa":"0.1"}

參考：

• https://www.php.net/manual/en/ini.core.php
  PHP: Description of core php.ini directives - Manual
• https://bugs.php.net/bug.php?id=74221
  PHP :: Bug #74221 :: Float precision is altered when encoding to JSON for floats 0.99... and under
• https://stackoverflow.com/questions/42981409/php7-1-json-encode-float-issue
  php - PHP7.1 json_encode() Float Issue - Stack Overflow
• https://wiki.php.net/rfc/precise_float_value
  PHP: rfc:precise_float_value
• https://www.jianshu.com/p/44d0855da415
  2019-11-13 PHP7.x  json_encode() Float Issue - 简书