Why Does PHP's Referenced Foreach Loop Exhibit Unexpected Behavior?

Understanding the Strange Behavior of PHP's Referenced Foreach Loop

In PHP, the foreach loop iterates over elements in an array or object. However, when using a reference within the loop (e.g., foreach ($a as &$v) { ... }), unexpected behavior can occur.

The Problem

Consider the following PHP script:

$a = array('a', 'b', 'c', 'd');

foreach ($a as &$v) { }
foreach ($a as $v) { }

print_r($a);

Expected Output:

An array with elements 'a', 'b', 'c', and 'd'.

Actual Output:

Array
(
    [0] => a
    [1] => b
    [2] => c
    [3] => c
)

Explanation

The unexpected behavior arises because references created within the foreach loop persist after the loop terminates. Specifically, the last element in the $a array may still be referenced by the variable $v.

When the second foreach loop runs, $v (a reference to the last element in $a) is reassigned the value of each element in the array. Since $v is a reference to the last element in $a, the corresponding array element is modified.

Resolution

To prevent this behavior, always unset the reference variable after the foreach loop completes:

$a = array('a', 'b', 'c', 'd');

foreach ($a as &$v) { }
unset($v);
foreach ($a as $v) { }

print_r($a);

Output:

Array
(
    [0] => a
    [1] => b
    [2] => c
    [3] => d
)

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