This is targeting 8.0.
`$arg` seems like a poor choice of a name,
especially if the function were to have arguments added.
In many cases, the php.net documentation already has $array for these functions.
E.g. https://www.php.net/manual/en/function.array-intersect.php
I'd assume that since named arguments was added to 8.0 near the feature freeze,
PHP's maintainers had planned to make the names consistent
and gradually use the same name for docs and implementation.
Otherwise we're missing the "expected to be a reference, value
given" warning that appears for ordinary calls to call_user_func().
Also update an UPGRADING note with recent changes wrt
call_user_func().