Bug #66450 [Nab]: Casting E notation strings to floats results in incorrect value

Edit report at https://bugs.php.net/bug.php?id=66450&edit=1 ID: 66450 Updated by: requinix@php.net Reported by: howard at zedcore dot com Summary: Casting E notation strings to floats results in incorrect value Status: Not a bug Type: Bug Package: *Programming Data Structures Operating System: Debian Gnu/Linux "Wheezy" 7.2 PHP Version: Irrelevant Block user comment: N Private report: N New Comment: To be clear it's the precision that matters here, but if that's a concern then you should consider the impact of serialize_precision too. Previous Comments: ------------------------------------------------------------------------ [2014-01-09 18:34:43] requinix@php.net Floating point values have a limited precision. Hence a value might not have the same string representation after any processing. That also includes writing a floating point value in your script and directly printing it without any mathematical operations. If you would like to know more about "floats" and what IEEE 754 is, read this: http://www.floating-point-gui.de/ Thank you for your interest in PHP. The exact values depend on your precision and serialize_precision settings. $ php -i | grep precision precision => 14 => 14 serialize_precision => 17 => 17 $ php -r 'var_dump(...);' float(2.6881171418161E+43) string(19) "2.6881171418161E+43" bool(false) float(3.5652673131419E+29) $ php -d precision=20 -d serialize_precision=25 -r 'var_dump(...);' float(2.6881171418161356094E+43) string(25) "2.6881171418161356094E+43" bool(true) float(0) ------------------------------------------------------------------------ [2014-01-09 17:46:31] howard at zedcore dot com It seems to affect any E notation string being cast to float. var_dump(exp(10), (string)exp(1), exp(1) == (string)exp(1), exp(1) - (string)exp(1)); Result: float(2.718281828459) float(2.718281828459) bool(false) float(4.5297099404706E-14) ------------------------------------------------------------------------ [2014-01-09 17:40:43] howard at zedcore dot com Description: ------------ Casting a very large float into a string and back-again results in a different value. The string representation of both values from var_dump() is the same, and as expected. Reproducible on PHP 5.4.4 and PHP 5.3.10 (Not able to upgrade to the latest PHP release at work but I'll try to reproduce at home soon). Test script: --------------- var_dump(exp(100), (string)exp(100), exp(100) == (string)exp(100), exp(100) - (string)exp(100)); Expected result: ---------------- float(2.6881171418161E+43) float(2.6881171418161E+43) bool(true) float(0) Actual result: -------------- float(2.6881171418161E+43) float(2.6881171418161E+43) bool(false) float(3.5652673131419E+29) ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=66450&edit=1