Addon Domain - PHP Error

I have created an addon domain and pointed the domain to my shared cPanel hosting. The change was a success and I could see my homepage, after uploading to the subdomain directory. However, beyond the homepage I have a 'litecart' installation which uses VQMod in PHP and this is creating an error:

Fatal error: VQMod::bootup - vQmod requires the PHP DOMDocument extension in /home/.........

I could not find any relative documentation on this other than this feature should be default ON with PHP 5.6 and beyond. I am using 7.2.

Not to be outdone, I changed the actions of my addon domain to look at an identical domain, which is my default domain which is running exactly the same files/site. I did this by changing the root folder of the addon domain - still errors.

I then went deeper and in the .system-php folder, I created a folder for the addon domain and added the same files, which includes the dom.ini file and has the following:


; Enable dom extension module
extension=dom.so

Still no joy.

So, in summary. I have a main domain on cpanel and an addon domain. Both are running the exact same pages, yet one works with VQMod and one errors with it. Where am I going wrong?