I am so grateful for everyone’s help in this - but you are all overthinking it 
(same as related post)
Is it possible to include a file using PHP but NOT the other files that it includes
I do appreciate that many posters are not thinking of all the options and there are better ways of doing things and it is great that we can offer other solutions, but my problem was in each case simply what the question asked.
I am not using Wordpress, I am not connecting to a database, I do not want a functioning config file I simply want to extract the vars from the initial wp-config, for reporting and analysis. There is a Wordpress installed but I am not interested in running it, changing the db, switching dbs creating alternative configs. I really just want to do exactly what the questions ask. The ultimate goal is just to extract the vars and their values.
But seriously - thanks to all as always it is by reading your suggestions and ideas that I finally arrived at a solution that meets my needs.