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Bug #73169 Variable becames reference
Submitted: 2016-09-25 16:57 UTC Modified: 2016-10-03 20:51 UTC
From: kilobyte at freemail dot hu Assigned:
Status: Not a bug Package: *General Issues
PHP Version: Irrelevant OS: Irrelevant
Private report: No CVE-ID: None
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 [2016-09-25 16:57 UTC] kilobyte at freemail dot hu 
Description:
------------
the code:

class CC
{
    private static $bt;

    public function run(&$a)
    {
        self::$bt[] = debug_backtrace();
        $a = 222;
    }
}

function n(&$a)
{
    (new CC())->run($a);
}

$a = [
    'a' => 1,
    'b' => 2,
    'c' => 3
];

n($a['b']);
echo '<pre>';
var_dump($a);

--------------------

this generates the following:

array(3) {
  ["a"]=>
  int(1)
  ["b"]=>
  &int(222)
  ["c"]=>
  int(3)
}

there is a "reference &" signal at "b". If I remove this line: "self::$bt[] = debug_backtrace();" its gone, it will work right, but why do this variable goes reference?

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 [2016-09-25 19:21 UTC] yohgaki@php.net
-Operating System: Windows 7 +Operating System: Irrelevant -PHP Version: 5.6Git-2016-09-25 (snap) +PHP Version: Irrelevant
 [2016-09-25 19:21 UTC] yohgaki@php.net 
https://3v4l.org/ZLToT
All versions do the same. HHVM seems to have problem with this.
 [2016-10-03 20:51 UTC] bwoebi@php.net
-Status: Open +Status: Not a bug
 [2016-10-03 20:51 UTC] bwoebi@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

debug_backtrace() is storing the arguments of the stackframes as is, i.e. it retains references too.

Thus CC::$bt[0][0]["args"]["b"] will be another variable pointing to $a["b"].

If you remove this debug_backtrace() call, then, obviously, all the other variables pointing to $a["b"] have been destroyed and thus there's no reference anymore.
Using DEBUG_BACKTRACE_IGNORE_ARGS as first parameter to debug_backtrace() will have the same effect.
 
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