Referenced array iteration in php results in strange result

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.