Edit report at https://bugs.php.net/bug.php?id=29992&edit=1
ID: 29992
Comment by: email at stevemann dot net
Reported by: fletch at pobox dot com
Summary: foreach by reference corrupts the array
Status: Not a bug
Type: Bug
Package: Scripting Engine problem
Operating System: linux
PHP Version: 5.0.1
Block user comment: N
Private report: N
New Comment:
Agreed this is not a bug, it's expected behaviour. But it's dangerous as it can
slip by without being noticed. It almost certainly means there are
thousands of sites which are exhibiting wrong behaviour because of this and
no-one realises. Surely the concept of scoping the 'as' variable to the
foreach enclosure only can't be considered bad form. It would make so much more
sense to 'opt-in' to retrieving the variable outside of the enclosure
(by assigning to another persistent variable within the enclosure) rather than
the current 'opt-out' system (using unset()) which, unless you happen
to have read the warning is HIGHLY DANGEROUS.
Previous Comments:
------------------------------------------------------------------------
[2013-05-20 15:57:18] paul dot dillinger at gmail dot com
OK, I went over this some more.
<pre>
<?php
// Fresh array
$clean = array(1,2,3,4);
foreach($clean as &$item){
// Nothing is modified in the array, but $item now exists
}
/*##############################################################################
* $item persists outside of foreach and is now $clean[3]
* See the warning on http://php.net/manual/en/control-structures.foreach.php
* print_r($item); // would return 4 you you uncommented this.
* unset($item); // This would remove the pointer to $clean[3]. Expected.
*############################################################################*/
echo "A:\n";
/*A*/ print_r($clean); // $clean is still unmodified
echo "B:\n";
foreach($clean as $item){
/*##############################################################################
* Using AS $item SETS $item TO the current $item value (a.k.a. $clean[0], etc.)
* Essentially foreach($clean as $item) is short hand for something like:
* $x=0;while($x < count($clean)){$item=$clean[$x]; ### your code ### $x++;}
* The problem I had was that I did not expect foreach to be able to set on call
*############################################################################*/
/*B*/ print_r($clean);
}
?>
</pre>
So creating the variable is documented, and it isn't a bug.
The ability to set the value could be made clearer though.
------------------------------------------------------------------------
[2013-05-20 09:51:33] richard at ejem dot cz
This IS a bug and SHOULD be finally fixed. It is weird, hard to debug and
unexpected behavior which took me many hours of life finding the problem.
Almost every modern programming language has a variable context, which
guarrantees the foreach variable is not visible after exiting the loop.
Anyone is using it intentionally for "weird reason"? come on guys, almost every
bug can be used for some hacking or weird reasons, will it stop you from fixing
other bugs? no!
Sorry for "mee-to" like post, but I do it for the good of next PHP programmer
generations who will also lose hours of their lives discovering this nonsense.
Please fix it.
------------------------------------------------------------------------
[2012-10-25 17:01:03] paul dot dillinger at gmail dot com
I understand that my explanation above isn't 100% accurate (unset really
doesn't
have anything to do with it), but that doesn't change that the expected
behavior
is not working.
Rasmus said:
"Ok, simple question then. Do you expect this to output 3?
foreach(array(1,2,3) as $b) { }
echo $b;
If you do, then you don't want us to fix this "bug"
because fixing it would mean $b is not 3 here."
I say in the example above, would you expect a print_r of the same array to
return 1,2,2? My issue was that the content of the entire array.
Honestly I've been programming since I was eight years old. That was 1985. If
this was confusing the hell out of me then something's wrong.
Here's an even simpler example:
<pre>
<?php
$clean = array(1,2,3,4);
foreach($clean as &$item){
// Nothing
}
echo "A:\n";
/*A*/ print_r($clean);
echo "B:\n";
foreach($clean as $item){
/*B*/ print_r($clean);
}
$clean = array(1,2,3,4);
foreach($clean as &$item){
// Nothing
}
echo "C:\n";
/*C*/ print_r($clean);
echo "D:\n";
foreach($clean as $not_item){
/*C*/ print_r($clean);
}
?>
</pre>
A and B output the same array differently with no modification (not expected).
C and D are the same (expected). The only change was not re-using the name
$item.
How can we make it so that using &$item in one foreach and then using the same
variable name ($item) in a different foreach does not change the original array?
------------------------------------------------------------------------
[2012-10-25 15:49:27] paul dot dillinger at gmail dot com
I still say this is a bug, so here's all the code you need to re-produce it:
<pre>
<?php
$packages = array();
$library_names = array();
$ext_js =
array(
array(
'name' => 'myname1',
'attrib' => 'something1',
'package' => true,
'library_name' => 'package1'
),
array(
'name' => 'myname2',
'attrib' => 'something2',
'package' => true,
'library_name' => 'package1'
),
array(
'name' => 'myname3',
'attrib' => 'something3',
'package' => false
),
array(
'name' => 'myname4',
'attrib' => 'something4',
'package' => false
)
);
foreach($ext_js as &$item){
if(!empty($item['package'])){
$packages[] = $item; // Not using &, so should be a copy, not a
reference corrent?
$library_names[] = $item['library_name'];
unset($item);
}
}
if(!empty($packages)){
/*A*/ print_r($ext_js);
foreach($packages as $item){
/*B*/ print_r($ext_js);
}
}
?>
</pre>
Look at the output on the last item. Instead of unset removing the item from
the array $ext_js (which is what I thought it would do), it corrupts the array.
The array is fine though unless you go in to another foreach using another
$item.
Changing the variable name on the second foreach to something OTHER than $item
(I used $itemm) fixes it.
Bug.
------------------------------------------------------------------------
[2012-10-25 07:37:08] email at stevemann dot net
I don't think this is going to go anywhere - seems to have reached a stalemate.
So I have just retrained my head to automatically create foreach loops thus:
foreach($array as $item){
}unset($item);
If you need access to the last $item outside the loop, then just do it
somewhere before the unset($item).
Seems to me this thread is being accessed periodically by developers scratching
their heads after discovering similar oddities happening with their foreach
loops. My advice would be to do something similar to the above and just live
with it.
------------------------------------------------------------------------
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the rest of the comments, please view the bug report online at
https://bugs.php.net/bug.php?id=29992
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