I'm sure I'm missing something obvious here.
I want to include a php file but I need to check if it exists first and every time I run it, I get a failure message. I can see the file is there from my FTP application.
That did it Fred. I've never had to do that before. Post it as an answer and I'll upvote it and accept it. – Richard Young
Posting my comment as "the" answer:
Use a full server path (an example):
or a relative one.