Why Does Using References in PHP's `foreach` Loop Lead to Unexpected Array Modifications?

Reference Behavior in 'foreach' Loop: Understanding Array Modifications

In PHP, employing references within a 'foreach' loop can lead to unexpected array modifications. This behavior is highlighted in the following code:

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

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

print_r($a);

The output this code produces is unexpected:

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

Step-by-Step Explanation

To understand this behavior, it's essential to follow the changes that occur during each iteration of the 'foreach' loop:

• 1st Iteration (Reference Iteration): $v is a reference to $a[0] ('a').
• 2nd Iteration (Reference Iteration): $v is a reference to $a[1] ('b').
• 3rd Iteration (Reference Iteration): $v is a reference to $a[2] ('c').
• 4th Iteration (Reference Iteration): $v is a reference to $a[3] ('d').

Upon completion of the first 'foreach' loop (reference iterations), $v still holds a reference to $a[3] ('d').

• 1st Iteration (Value Iteration): $v (still referencing $a[3]) is assigned the value of $a[0] ('a'). However, since $v is a reference, it modifies $a[3] to 'a'.
• 2nd Iteration (Value Iteration): $v (still referencing $a[3]) is assigned the value of $a[1] ('b'). This again modifies $a[3] to 'b'.
• 3rd Iteration (Value Iteration): $v (still referencing $a[3]) is assigned the value of $a[2] ('c'). This modifies $a[3] once more to 'c'.
• 4th Iteration (Value Iteration): $v (still referencing $a[3]) is assigned the value of $a[3] ('c').

Thus, after the second 'foreach' loop (value iterations), the array $a has been modified with 'c' appearing twice.

Resolving the Issue

To avoid this unexpected behavior, it is recommended to unset the reference after each iteration:

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

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

print_r($a);

This will yield the expected output:

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

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