PHP7.1 json_encode() Float Issue

Question

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 cha

Answer 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}