php list() undefined behaviour

Question

I came across this line:

list($diff, $current, $concurrent) = $diff;

Documentation states that this should result in undefined behaviour. What are the possible variants of this behaviour? Variable $diff is array, containing 3 elements with variable content.

This line is part of application that contains a bug and author of this line is unavailable. Though I am almost sure that it is not what I am looking for, it would be nice to be 100% sure. I am using PHP 5.6.25 as FPM/FastCGI. Thanks in advance.

Can you edit the application code? If so $array = $diff; list($diff, $current, $concurrent) = $array; will eliminate any undefined behavior. — Steve
– Steve, Commented Nov 24, 2016 at 14:05
It's undefined for a reason, it's likely to have a different effect depending on your version/operating system etc. You are overwriting $diff with $diff whilst it's been evaluated. Perhaps doing what @Steve suggested or changing the list($diff... variable will help. — R. Chappell
– R. Chappell, Commented Nov 24, 2016 at 14:07
Would be nice if I got 2 more comments on improving my question, instead of 2 downvotes. — svgrafov
– svgrafov, Commented Nov 24, 2016 at 15:23
@Steve Of course I will remove potentially undefined behaviour, but I would like to know the exact possibilities. Hope I am not asking for too much. — svgrafov
– svgrafov, Commented Nov 24, 2016 at 15:25
@R.Chappell Can you give a couple of actual examples on how can this or similar thing work differently? — svgrafov
– svgrafov, Commented Nov 24, 2016 at 15:27

Striezel · Accepted Answer · 2016-11-24 14:09:23Z

As the documentation for list() also states:

In PHP 5, list() assigns the values starting with the right-most parameter. In PHP 7, list() starts with the left-most parameter.

In other words: This line might work as intended in PHP 5, because the variable $diff that appears on both sides is the last variable to get assigned. However, in PHP 7 the $diff variable gets assigned first, so $diff has already changed by the time the assignments for $current and $concurrent are done.

In general I think the hint about undefined behaviour relates to the fact that you cannot rely on certain assignments to yield the expected results, if a variable appears on both sides of the = sign. A workaround for the issue could look like this:

list($temp, $current, $concurrent) = $diff;
$diff = $temp;
unset($temp);

This way you avoid the undefined behaviour.

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