The application I have built is for bulk email. It is compatible with all PHP5 versions, but not PHP7.
Upgrading the project to PHP7 would be a big job, maybe for the future.
The application can run on any server with PHP 5 available. Would it be Ok to sell for PHP5 only? This would require users with main site running a PHP 7 application to load the app to a subdomain running PHP5 and MySQL 5+.