I think we’d have to see the file you’re trying to include to say for certain. Could be a character encoding mismatch to the source you’re trying to include from – could be you’ve got php in that .html (which WILL execute if you use include)
Include is for code – have you tried using readfile instead? Readfile is for when you do not want the possibility of code execution… include/require is for when you want the file (regardless of it’s extension) to be checked for and allowed to execute php.
That’s weird. Sounds like there’s more than meets the eye here. All the same, you could perhaps replace the ’ with a corresponding entity reference if you want a quick fix: '