Rename standard array function parameters to $array

Rename standard array function parameters to $array

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.