Bug #79498 [Com]: Modifying and evaluating a variable in the same expression

Edit report at https://bugs.php.net/bug.php?id=79498&edit=1 ID: 79498 Comment by: maple_ at outlook dot com Reported by: maple_ at outlook dot com Summary: Modifying and evaluating a variable in the same expression Status: Not a bug Type: Bug Package: *General Issues Operating System: linux PHP Version: master-Git-2020-04-20 (Git) Block user comment: N Private report: N New Comment: i finally know whats means 'undefined behavior', thank u! Previous Comments: ------------------------------------------------------------------------ [2020-04-20 03:07:11] requinix@php.net > $a[$b+1]=$b=2; Trying to modify a variable and use it in the same statement results in undefined behavior, meaning PHP makes no promises about whether the $b=2 evaluates before or after the $b+1. If you want to find out more, learn about "sequence points". Most information will be about C/C++ but it applies PHP as well. ------------------------------------------------------------------------ [2020-04-20 03:00:22] maple_ at outlook dot com Description: ------------ The following two pieces of code have the same output. i want to know is that the correct feature of php or something? Test script: --------------- $b=1; $a=array(1=>"maple"); $a[$b+1]=$b=2; var_dump($a); $b=1; $a=array(1=>"maple"); $a[$b]=$b=2; var_dump($a); Expected result: ---------------- array(2) { [1]=> string(5) "maple" [3]=> int(2) } array(2) { [1]=> string(5) "maple" [2]=> int(2) } Actual result: -------------- array(2) { [1]=> string(5) "maple" [2]=> int(2) } array(2) { [1]=> string(5) "maple" [2]=> int(2) } ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=79498&edit=1