php foreach array_merge problems and confusion

Code:

<code>$a = array('a' => array('a'));
$b = array('b' => array('b'));

foreach ($b as $key => &$item) {
    $item[] = 'd';
}
$a = array_merge($b, $a);
$b['b'][] = 'c';
print_r($a);
print_r($b);</code>

Result:

<code>Array
(
    [b] => Array
        (
            [0] => b
            [1] => d
            [2] => c
        )
    [a] => Array
        (
            [0] => a
        )
)
Array
(
    [b] => Array
        (
            [0] => b
            [1] => d
            [2] => c
        )
)</code>

My confusion is why the operation on $b affects $a after the merger.
Anyone who knows, please clarify, thank you!

Reply content:

Code:

<code>$a = array('a' => array('a'));
$b = array('b' => array('b'));

foreach ($b as $key => &$item) {
    $item[] = 'd';
}
$a = array_merge($b, $a);
$b['b'][] = 'c';
print_r($a);
print_r($b);</code>

Result:

<code>Array
(
    [b] => Array
        (
            [0] => b
            [1] => d
            [2] => c
        )
    [a] => Array
        (
            [0] => a
        )
)
Array
(
    [b] => Array
        (
            [0] => b
            [1] => d
            [2] => c
        )
)</code>

My confusion is why the operation on $b affects $a after the merger.
Anyone who knows, please clarify, thank you!

Actually, I think you can see it if you change print_r to var_dump

<code>array (size=2)
  'b' => &
    array (size=3)
      0 => string 'b' (length=1)
      1 => string 'd' (length=1)
      2 => string 'c' (length=1)
  'a' => 
    array (size=1)
      0 => string 'a' (length=1)
array (size=1)
  'b' => &
    array (size=3)
      0 => string 'b' (length=1)
      1 => string 'd' (length=1)
      2 => string 'c' (length=1)</code>

Variable b points to a reference type, so if the value of b is changed, the value of b in a will also change

Thank you for the reply from @风咷horn. I googled “php foreach reference” again and found this article, which gave me a better understanding of this issue

[PHP] An interview question about variable reference in foreach loop

Let me tell you my understanding. When using the & symbol, the variables $b and $item will point to an address at the same time. At this time, $b becomes a reference variable. If $item is unset after foreach, this will The reference of $b will be dereferenced.