User

Just like normal Meta Boxes, you can add meta fields for users.

PHP

$usermeta = register_cuztom_user_meta( $id, $data, $locations )

$id
(string) The ID of the Meta (box). Used for technical stuff

$data
(array) The data of the Meta Box

$locations
(string|array) The location the Meta Box is shown. Can be (and defaults to): 'show_user_profile', 'edit_user_profile'