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Bug #78172 Wrong rounding up/down when decimal places are specified
Submitted: 2019-06-16 16:18 UTC Modified: 2019-06-16 17:04 UTC
From: vecera at zdenekvecera dot cz Assigned: cmb (profile)
Status: Not a bug Package: *Math Functions
PHP Version: 7.1.30 OS: Windows, Linux
Private report: No CVE-ID: None
View Developer Edit
 [2019-06-16 16:18 UTC] vecera at zdenekvecera dot cz 
Description:
------------
var_dump(round(1.44444, 3, PHP_ROUND_HALF_DOWN)); // float(1.444) OK 
var_dump(round(1.55555, 3, PHP_ROUND_HALF_DOWN)); // float(1.556) BUG (correct value: 1.555)
var_dump(round(1.77777, 3, PHP_ROUND_HALF_DOWN)); // float(1.778) BUG (correct value: 1.777)

var_dump(round(1.44444, 3, PHP_ROUND_HALF_UP)); // float(1.444) BUG (correct value: 1.445)
var_dump(round(1.55555, 3, PHP_ROUND_HALF_UP)); // float(1.556) OK
var_dump(round(1.77777, 3, PHP_ROUND_HALF_UP)); // float(1.778) OK

// PHP version: 7.0.33, 7.1.30, 7.2.19, 7.3.6

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 [2019-06-16 17:00 UTC] danack@php.net 
I don't understand what how you get your expected values.

// round(1.44444, 3, PHP_ROUND_HALF_UP) (correct value: 1.445)

That would be the same as expected 0.00044 rounded to 3 digits to be 0.001. but 0.00044 is less than 0.0005 which is the halfway point to 0.001.


Similarly 0.00055 rounded to 3 figures is going to be 0.001 no matter the rounding.
 [2019-06-16 17:04 UTC] cmb@php.net
-Status: Open +Status: Not a bug -Assigned To: +Assigned To: cmb
 [2019-06-16 17:04 UTC] cmb@php.net 
Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

It's like @danack said; IOW the PHP_ROUND_HALF_* constants are
only relevant when the fractional part of $val * 10**$precision is
exactly 0.5.
 
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