It is confusing/wrong in this piece of the example code ("C\contains($items, 'a'); // true"). After modifying in above lines, it should be false
When explaining the immutable characteristic, I suppose it means the modification does not occur at the same memory location referenced Hack array, but instead create a modifiable copy. Perhaps it can use an example so it first assign the array to another variable ($items2 = $items) and then modify the $items, and check the values for both $items and $items2
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