WordPress - Fix missing or empty author dropdown

update_wp_user_level

// Update wp_user_level values to fix empty author dropdown
// Good explanation of the problem at http://ihisham.com/2014/05/wordpress-fix-missing-users-from-authors-dropdown-list/

// Get all users
$users = get_users();

foreach ( $users as $user ) {
	// Define user level by user role
	switch ( array_shift( $user->roles ) ) {
		case 'administrator':
			$user_level = 10;
			break;
		case 'editor':
			$user_level = 7;
			break;
		case 'author':
			$user_level = 2;
			break;
		case 'contributor':
			$user_level = 1;
			break;
		default:
			$user_level = 0;
	}

	// Update user level
	update_user_meta( $user->ID, 'wp_user_level', $user_level );
}