"Add Existing User" form does not preserve input in case of an error

[SergeyBiryukov]

Background: #27006

In single site, "Add New User" form preserves entered values in case of an error.

In Multisite, "Add New User" form preserves the values, but "Add Existing User" does not.

In network admin, "Add New User" form does not preserve the values.

[helen]

We should of course do this on the server side, but when autocomplete is in use, we really should only allow selection of an option and not the submission of freeform typing. See my comment on #19867.

[boonebgorges]

27606.patch fixes the issue.

The case of Network Admin > Add New User was straightforward. The case of Multisite > Add New User was a bit different, because a wp_redirect() takes place after the error, which means we lose the values submitted in $_POST. I've opted to add them as URL parameters when redirecting.

when autocomplete is in use, we really should only allow selection of an option and not the submission of freeform typing

Yes, but that's a bigger fish to fry, so I wrote a fix for the current implementation.

[SergeyBiryukov]

foreach ( array( 'username', 'email' ) as $post_key ) {
    $$post_key = isset( $_POST['user'][ $post_key ] ) ? stripslashes( $_POST['user'][ $post_key ] ) : '';
} 

Should probably avoid variable variables, see [28736] and other commits.

[SergeyBiryukov]

Related: #41314

[SergeyBiryukov]

Related: #47064