Over the years? Yes I've altered the php quite a bit to fit my needs, but not currently.
Hmm, did your hosting provider change the PHP version on your server/hosting package? Which PHP version does your server run?
The problem is I don't know what's missing. My error log shows a line that I can't seem to figure out over and over it's 19.
Can you provide an example of the error message(s)? Please anonymise pathes but leave errors, file names and line numbers.
I don't want to lose my database or all of the design or alteration work I've done to it over the years, but I'm really lost.
Ok., the forum content is stored in the database and in all probability the database itself is not affected by your problem. If available, you can inspect the database content in a tool like phpMyAdmin. Also the (currently non functioning) script files and theme changes are present on the server and you can make a backup whith your FTP client.
It was an older version, I haven't upgraded since I installed the latest version in 2014.
Then it seems to be a MLF version of the 2.3 branch but before 2.3.5. Which version do you run?