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 andserialize()
/var_export()
uses PG(serialize_precision) which set to 17 be default to be more precise. Sincejson_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}