Description:
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When the key of an array is assigned an exact same value, copy-on-write is triggered, while this could be avoided.

By "exact same", I mean "exact same memory address" - not "===" (which could require an array traversal to run the comparison).

Here is an example in Symfony where this logic is implemented in userland (but has to use `===` because there is no other way to do so in userland), and where it makes a difference when measuring the memory usage:
https://github.com/symfony/symfony/blob/4a602ca6529090197e09555796b27500111aee31/src/Symfony/Component/DependencyInjection/Compiler/AbstractRecursivePass.php#L60-L62

Test script:
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$a = $someExistingArray;

foreach ($a as $k => $v) {
    // [...] conditionnaly process $v, let's say it were unaffected;
    // then reassigning triggers COW, even if $v points to the exact same memory address/object

    $a[$k] = $v;
}

Expected result:
----------------
no COW