How to json_encode float values in PHP 7.1.1?

Question

PHP seems to have a bug in the way it handles decimal precision in json_encode.

It's easy to see just by encoding a simple float:

echo json_encode(["testVal" => 0.830]);

// Prints out:
{"testVal":0.82999999999999996003197111349436454474925994873046875}

I'm not much of a server admin, so aside from going into the php.ini and changing serialize_precision to -1, is there anything I can do in my code to protect against this when I can't be sure it's running in an environment where that setting has been changed?

EDIT: I'm sure some comments will want to link against general discussions of why floating point imprecision exists. I know that. My question here is specifically about the best practice for dealing with it in PHP, and whether there is a way to code defensively against it. Surely there is a better way than sending floats as strings.

As the discussion on the bug report you linked to makes clear, this is not a bug but a deliberate feature. If your server is configured to serialize floating point numbers with high precision, json_encode will now respect this. — IMSoP
– IMSoP, Commented May 9, 2017 at 9:22
And if you write that value as string? Then you have exact that number you want. — René Höhle
– René Höhle, Commented May 9, 2017 at 9:24
0.830 cannot be represented exactly as floating point. It is an approximation. PHP provides a better approximation (with a lot of decimal places). That's all. You can format the value as string using number_format() and put the string in JSON. Or, better, you can let it be a floating point number and do the formatting just before you want to put the value on screen. — axiac
– axiac, Commented May 9, 2017 at 9:30

Valery Viktorovsky · Accepted Answer · 2017-05-09 09:30:03Z

13

You should configure 'precision' and 'serialize_precision' params.

precision = 14
serialize_precision = -1

Test case:

php -r 'ini_set("precision", 14); ini_set("serialize_precision", -1); var_dump(json_encode(["testVal" => 0.830]));'
string(16) "{"testVal":0.83}"

answered May 9, 2017 at 9:30

Valery Viktorovsky

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5 Comments

Just a student Over a year ago

It might be helpful to link to the documentation changelog, where it is documented what changed. In particular, changing the precision no longer has an effect on json_encode.

Valery Viktorovsky Over a year ago

@Justastudent Thanks, thats true for 7.1, many guys still use 5.6 so answer above will be useful for everybody who use 5.6+.

GoldenJoe Over a year ago

Yeah, I mentioned this in my original question. Is this the best solution, and if so, why? Are there any alternatives?

Valery Viktorovsky Over a year ago

@GoldenJoe I think serialize_precision = -1 is more convenient than using number_format across the code. It's possible to create .user.ini in site public dir and then add serialize_precision = -1 option to it.

texelate Over a year ago

Although not without its limits, a simple fix is to cast the value as a string. See my answer: stackoverflow.com/a/50944401/2338825

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