Looping in array changes element type

Consider this PHP code snippet which loops in array by value reference:

$arr = [1 ,2 , 3];
var_dump($arr);
echo '<br>';

foreach ($arr as &$val) { // note ampersand sign
   // Anything
}

var_dump($arr);

Now first var_dump() emits

array(3) { [0]=> int(1) [1]=> int(2) [2]=> int(3) }

But second emits

array(3) { [0]=> int(1) [1]=> int(2) [2]=> &int(3) }

Thus looping in array by reference has changed last value type - from integer to integer reference ! How can it be at all ? Why PHP has decided itself to change element types without developer intention to do so ?

Solution

To understand this behaviour, you need to understand a few things about PHP:

References in PHP are symmetrical: you don't "create a reference to a variable", you "add the variable to a reference set".
A foreach by reference is the same as a series of assignments by reference, one after the other.
Array values are treated as variables in their own right, and can have all the same type information, including being part of a reference set.

So let's "unroll" your loop:

// create a reference set containing $val and $arr[0]
$val =& $arr[0];
// remove $val from the first reference set, 
// and create a second reference set containing $val and $arr[1]
$val =& $arr[1];
// remove $val from the second reference set, 
// and create a third reference set containing $val and $arr[2]
$val =& $arr[2];

At this point, $arr[0] and $arr[1] are each in a reference set of size 1, so can be seen as "normal values". However, $arr[2] is still in a reference set with $val.

What that means is that any change to $arr[2] will be reflected in $val, and any change in $val will be reflected in $arr[2]. That's why var_dump is annotating that item with an &, to show that changing it will also change another variable somewhere else.

This is why it is a good habit to always run unset($val); after using a foreach-by-reference.