phpVMS 2.x - has been updated to work with PHP versions up to (7.1 or 7.2 maybe) - sometimes (incorrectly) referred to as version 5
phpVMS 5.5.2 - for PHP version 5 up to 5.6 - Simpilot version
phpVMS 5.5.2.72 - this is phpVMS 5.5.2 (Simpilot version) updated to work with PHP 7.0-7.2 - will NOT presently work under PHP 7.3
phpVMS 7 - still in beta testing - not compatible with prior versions - probably not ideal to use in a live environment at this point - will work in PHP 7.3
The version you have installed will not work with PHP 7.3 - can you revert PHP to 5.6, 7.0, 7.1 or 7.2?
If you install phpVMS 5.5.2 or 5.5.2.72, you will need to select MySQLi (Extended) for the database - MySQL will not work. I believe MariaDB 10.1 will also work - but not MariaDB 10.2
Let us know if you can revert to another version of PHP and also what name and version of database you are using.
That error is probably because you are using mysql instead of mysqli (extended) - you may have been given a choice during installation, you need to choose mysqli (extended).