At first, it wasn't working because httpd reported:
[:error] [pid 1807] [client] PHP Fatal error: Class 'MarkdownExtra_Parser' not found in /var/www/html/wiki/extensions/MarkdownExtraParser/MarkdownExtraParser.php on line 120
And I realized there was an underscore: class MarkdownExtraOverride extends MarkdownExtra_Parser {
so I took it out, but now I get a different error:
[:error] [pid 2524] [client ] PHP Fatal error: Call to undefined method MarkdownExtraOverride::transform() in /var/www/html/wiki/extensions/MarkdownExtraParser/markdown.php on line 55
So I'm not sure if it's using some deprecated functions or something. I know PHP markdown is nearly a 13 year old extension, so wasn't sure if it should still work. I'm using 5.6.