Where is my server substituting for a ### string?

I'm trying to move a PHP based website to a new ISP for a friend as the original developers have lost all of their PHP capability (and I'm really an Oracle programmer).

I've got a Dev environment set up on my home system using LAMP which is almost serving everything properly. I'm trying to emulate the prod environment, so have set the website up under its own directory on the server (not the base Apache directory), have set it up with its own example.com.conf file, and have simulated example.com in my /etc/hosts files on dev server and test client.

One of the templates that is included includes the line : <base href="### WEB_ROOT ###" />

Which is coming out of the original web server as : <base href="https://example.com/" />

Unfortunately it is coming out of my development server as : <base href="https://localhost/" /> Which means that none of the relative paths are working.

There are also other lines that use the same notation for TITLE, META DESCRIPTION and others.

I've tried searching for PHP, HTML and Apache substitution strings, but I can't find anything about this '###' notation.

I don't even know where in the stack this substitution is happening, so if someone can please point me at some documentation about it, and especially where I need to set the WEB_ROOT etc. variables, that would be wonderful.

Thanks