Referenced array iteration in php results in strange result - php

Question

I have seen strange behavior that I don't quite get. I do the following:
$array = [
'a' => [
'a1' => [
'a11' => 1,
'a12' => 2
],
'a2' => [
'a21' => 3,
'a22' => 4
],
],
'b' => [
'b1' => [
'b11' => 1,
'b12' => 2
],
'b2' => [
'b21' => 3,
'b22' => 4
],
],
];
foreach ($array as $strLevel1 => &$arrLevel1)
{
foreach ($arrLevel1 as $strLevel2 => &$arrLevel2)
{
foreach ($arrLevel2 as $strLevel3 => &$varLevel3)
{
$varLevel3 = 0;
}
}
}
echo '<pre>';
var_dump($array);
echo '</pre>';
foreach ($array as $strLevel1 => $arrLevel1)
{
}
echo '<pre>';
var_dump($array);
echo '</pre>';
The result is as follows:
array(2) {
["a"]=>
array(2) {
["a1"]=>
array(2) {
["a11"]=>
int(0)
["a12"]=>
int(0)
}
["a2"]=>
array(2) {
["a21"]=>
int(0)
["a22"]=>
int(0)
}
}
["b"]=>
&array(2) {
["b1"]=>
array(2) {
["b11"]=>
int(0)
["b12"]=>
int(0)
}
["b2"]=>
&array(2) {
["b21"]=>
int(0)
["b22"]=>
&int(0)
}
}
}
array(2) {
["a"]=>
array(2) {
["a1"]=>
array(2) {
["a11"]=>
int(0)
["a12"]=>
int(0)
}
["a2"]=>
array(2) {
["a21"]=>
int(0)
["a22"]=>
int(0)
}
}
["b"]=>
&array(2) {
["a1"]=>
array(2) {
["a11"]=>
int(0)
["a12"]=>
int(0)
}
["a2"]=>
array(2) {
["a21"]=>
int(0)
["a22"]=>
int(0)
}
}
}
As you can see, in the first output everything is ok. But in the second one, the b-named branch of the array is replaced by the a-named branch. This is because of the referencing I did. If I put a "&" before $arrLevel1 in the last loop, it works again.
Why is that? Am I doing something wrong with the references? Or should I don't use them at all and do array manipulation only fully qualified without any reference?
Thanks in advance.

Answer 1

Maybe you should unset the reference to $arrLevel1 after your loop, till you reused the var for the second loop.
foreach ($array as $strLevel1 => &$arrLevel1)
{
foreach ($arrLevel1 as $strLevel2 => &$arrLevel2)
{
foreach ($arrLevel2 as $strLevel3 => &$varLevel3)
{
$varLevel3 = 0;
}
}
}
// remove reference
unset($arrLevel1);
echo '<pre>';
var_dump($array);
echo '</pre>';
foreach ($array as $strLevel1 => $arrLevel1)
{
}
echo '<pre>';
var_dump($array);
echo '</pre>';
A way to come around unset is to use unique names for your loop variables or - if you want to loop over the same array again - just also use an reference loop var.
foreach ($array as $strLevel1 => &$arrLevel1)
{
foreach ($arrLevel1 as $strLevel2 => &$arrLevel2)
{
foreach ($arrLevel2 as $strLevel3 => &$varLevel3)
{
$varLevel3 = 0;
}
}
}
// also use a reference
foreach ($array as $strLevel1 => &$arrLevel1)
{
}
Most of the time, it might be simplier to just don't use references or move the loop in an "atomic" function which just does the manipulation and returns the result.