When you upgrade Dokuwiki (brilliant wiki BTW) it does it in-place. Unfortunately, I hadn’t upgraded for about 5 years in spite of numerous warnings so the deprecated.php file bit me by assuming there were specific files in my instance of dokuwiki that needed deprecating and breaking when the system was so old they weren’t ;-). Once I figured the problem out the solution was simply to comment out the appropriate parts of deprecated.php. After all, there is no need to deprecate something you don’t even have.
ini_set('display_errors', 1); ini_set('display_startup_errors', 1); error_reporting(E_ALL);
in index.php when trying to identify problems running a PHP app. And remove it afterwards.