Description:
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Copy array.

When arrays next index=2 and max key=2, next key is both 2.
When arrays next index=2 and max key=1, next key is both 2.
When arrays next index=2 and max key=0, next key is 0 or 2.


Test script:
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https://3v4l.org/TYc10

$a = [1, 2];
unset($a[1], $a[0]);
$b = $a;

$a[] = 3;
$b[] = 4;

var_dump($a, $b); // $a = [0=>3]  $b = [2=>4]


When increase variables, only the last one take over key.

Expected result:
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Always same key whth $a and $b.

Actual result:
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Same key when array is not empty.
Not same key when array is empty.