Weird PHP behaviour - What's going on? [duplicate] - php

Question

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Strange behavior Of foreach
Just came across this bug recently in a PHP app. Not sure what's going on.
Basically, it shows up when using a combination of two foreach (one with &, one without).
Here's a test code that reproduce the problem:
$items = array(
array('id'=>1, 'name'=>'foo', 'value'=>150),
array('id'=>2, 'name'=>'bar', 'value'=>190)
);
foreach($items as &$item)
{
$item['percentage'] = $item['value'] * 0.75;
}
var_dump($items); // All Good
foreach($items as $item)
{
var_dump($item); // Shows 1st item twice
}
The second foreach loop runs the block twice, as expected, but $item remains stuck on the first item.
I understand this is likely caused by the use of the reference & in the first loop but I don't see why it should behave like this..
Any idea? is that a bug?
Getting the same result on 5.3.8, 5.3.10 & 5.4

Answer 1

Firstly, it is not a bug as Rasmus said. See https://bugs.php.net/bug.php?id=29992
In this, right implementation of modifying array with its loop variable with &.
<?php
$arr = array(1, 2, 3, 4);
foreach ($arr as &$value) {
$value = $value * 2;
}
// $arr is now array(2, 4, 6, 8)
unset($value); // break the reference with the last element
var_dump($arr); // All Good
foreach($arr as $value) {
var_dump($value); // All good
}
?>

Answer 2

This is odd PHP behavior that's been around pretty much forever, and it happens when you mix the use of a variable as reference then not reference like you did.
I deal with it with a naming convention as follows: When I am using foreach with a &$item, I name it like &$refItem. This keeps me from mixing types.

Answer 3

You need to unset the pointer after using foreach with a referenced array.
http://php.net/unset

Answer 4

This may be something that look more understand the problem of foreach
$last_value_of_first_foreach = 1;
$item = 2;
$c = 3;
$item = &$last_value_of_first_foreach ; // Think that this statement is first foreach loop
// Here $item is pointer to $last_value_of_first_foreach
// To Better understanding, let change the name ($reference_to_last_value = $item;)
now, the new loop is
$item = $c;
// Here, what it do is update value where the $item pointer refer to
// (mean $last_value_of_first_foreach )
// so, at here $last_value_of_first_foreach has value of $c
Now, back to your case, from the first foreach, the $item reference to the last element of array. now, when you assign something to $item in second foreach, what it do, is put something inside that one.
In the end of first loop
$item is pointer to $items[1]
The first of second loop
it will push the first element to the location where the $item point to (that mean $items[1], so that why $items[1] is replaced by $items[0].
In case you want to prevent this one, just unset the $item variable before next time usage.

Answer 5

This is a normal, not a weird behavior. Just read about reference here.
When you add & in front of a variable, you store the reference to a variable. So, when you re-use it, it will also change the contents of the referenced variable.
foreach($items as &$item) // You have $item here, prefixed with &.
{
$item['percentage'] = $item['value'] * 0.75;
}
var_dump($items);
foreach($items as $item) // And your re-use it here.
{
var_dump($item);
}
To solve this, add unset($item) in the 1st loop:
foreach($items as &$item)
{
$item['percentage'] = $item['value'] * 0.75;
unset($item); // add this, because you use &$item previously.
}