Why does using pass-by-reference in a PHP `foreach` loop modify the array after the loop ends?

PHP Foreach Pass by Reference and Array Modification

In PHP, using pass by reference in a foreach loop can lead to unexpected behavior. Consider the following code:

$arr = ["foo", "bar", "baz"];

foreach ($arr as &$item) {}
// Array remains unchanged: ["foo", "bar", "baz"]

foreach ($arr as $item) {}
// Array is modified: ["foo", "bar", "bar"]

Why does the second loop modify the array?

In the first loop, the $item variable is passed by reference. This means that changes made to $item also affect the corresponding element in the $arr array. However, in the second loop, $item is passed by value. Thus, changes made to $item do not affect the array.

Crucially, after the first loop, $item still references the last element of $arr. When the second loop iterates over the array, each value of $item overwrites the last element of $arr because they both refer to the same memory location.

Debugging the Output

To understand the behavior, we can echo the current value of $item and recursively print the $arr array during each loop iteration.

First Loop:

foo
Array ( [0] => foo [1] => bar [2] => baz )

bar
Array ( [0] => foo [1] => bar [2] => baz )

baz
Array ( [0] => foo [1] => bar [2] => baz )

After the first loop, $item points to the last element of $arr.

Second Loop:

foo
Array ( [0] => foo [1] => bar [2] => foo )

bar
Array ( [0] => foo [1] => bar [2] => bar )

bar
Array ( [0] => foo [1] => bar [2] => bar )

As each value of $item is overwritten, it also modifies the last element of $arr.

Is it a Bug?

No, this behavior is not a bug but rather the intended behavior of pass-by-reference. It's important to understand the implications of passing variables by reference and to use it appropriately.

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