How to allow an user role to create a new user under a role which lower than his level only?

I have three extra roles in my site.

1. Doctor
2. Receptionist
3. Guest

those roles are added by following code:

* adding Doctor role */
$doctor_role = add_role('Doctor', __('Doctor'), array('read'=>'true'));

/* adding Receptionist role */
$receptionist_role = add_role('Receptionist', __('Receptionist'), array('read'=>'true'));

/* adding Guest role */
$guest_role = add_role('Guest', __('Guest'), array('read'=>'true'));

By default Administrator role create all other roles. But I want to limit this assigning role by user level.
What I mean is:

1. Administrator – should be able to create all role users – possible by default.
2. Doctor – should be able to create Receptionist and Guest role users ONLY
3. Receptionist – should be able to create Guest role users ONLY
4. Guest – Not allowed to create any users.

How can do this?
Better if I can achieve this without using any plugins.

Answers:

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Method 1

Firstly, you need to add the following capabilities to the Doctor and Receptionist role:

• list_users
• edit_users
• create_users
• delete_users

Now we can get to work with controlling which users they can create/edite/delete. Let’s start with a “helper” function that will return which roles a user is allowed to edit:

/**
 * Helper function get getting roles that the user is allowed to create/edit/delete.
 *
 * @param   WP_User $user
 * @return  array
 */
function wpse_188863_get_allowed_roles( $user ) {
    $allowed = array();

    if ( in_array( 'administrator', $user->roles ) ) { // Admin can edit all roles
        $allowed = array_keys( $GLOBALS['wp_roles']->roles );
    } elseif ( in_array( 'Doctor', $user->roles ) ) {
        $allowed[] = 'Receptionist';
        $allowed[] = 'Guest';
    } elseif ( in_array( 'Receptionist', $user->roles ) ) {
        $allowed[] = 'Guest';
    }

    return $allowed;
}

And to set which roles they can assign a user:

/**
 * Remove roles that are not allowed for the current user role.
 */
function wpse_188863_editable_roles( $roles ) {
    if ( $user = wp_get_current_user() ) {
        $allowed = wpse_188863_get_allowed_roles( $user );

        foreach ( $roles as $role => $caps ) {
            if ( ! in_array( $role, $allowed ) )
                unset( $roles[ $role ] );
        }
    }

    return $roles;
}

add_filter( 'editable_roles', 'wpse_188863_editable_roles' );

And finally, limit which users they can edit/delete based on their role:

/**
 * Prevent users deleting/editing users with a role outside their allowance.
 */
function wpse_188863_map_meta_cap( $caps, $cap, $user_ID, $args ) {
    if ( ( $cap === 'edit_user' || $cap === 'delete_user' ) && $args ) {
        $the_user = get_userdata( $user_ID ); // The user performing the task
        $user     = get_userdata( $args[0] ); // The user being edited/deleted

        if ( $the_user && $user && $the_user->ID != $user->ID /* User can always edit self */ ) {
            $allowed = wpse_188863_get_allowed_roles( $the_user );

            if ( array_diff( $user->roles, $allowed ) ) {
                // Target user has roles outside of our limits
                $caps[] = 'not_allowed';
            }
        }
    }

    return $caps;
}

add_filter( 'map_meta_cap', 'wpse_188863_map_meta_cap', 10, 4 );

Method 2

The above answer worked great. However, the roles need to be lowercase for

function wpse_188863_get_allowed_roles( $user ) { }

For instance:

if ( in_array( 'administrator', $user->roles ) ) { // Admin can edit all roles
    $allowed = array_keys( $GLOBALS['wp_roles']->roles );
} elseif ( in_array( 'doctor', $user->roles ) ) {
    $allowed[] = 'receptionist';
    $allowed[] = 'guest';
} elseif ( in_array( 'receptionist', $user->roles ) ) {
    $allowed[] = 'guest';
}

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0