Populates the last name if present in the first name field

lastname

add_filter( 'mailster_verify_subscriber', function( $entry ) {

	if ( isset( $entry['firstname'] ) ) {
		$names = explode( ' ', $entry['firstname'] );
		$entry['firstname'] = array_shift( $names );
		if ( ! empty( $names ) ) {
			$entry['lastname'] = implode( ' ', $names );
		}
	}

	return $entry;

} );