Regarding your suggestions, I’ve checked the conditions you listed and none seem to apply:

1. The permissions on some of the folders or files are 777 or 666. If this is the case, change them to either 755 or 644 in Cpanel's File Manager (or using your local FTP client).

[RCT] I have checked the permissions on folders and some files, and none are 777 or 666.  All directories were 755 and all files were 644.  I tried changing a few file permissions from 644 to 755 to see if would make a difference, but it did not. 

The ONLY directory that is not set to 755 is the “public_html” (not the home directory for the wiki), which is set to 750.  Is that as it should be?

2. The files and/or folders are not owned by you. Certain applications having been run under php as an apache module may have files owned by the apache user of nobody. An indication that you don't own the files would be if you are unable to change their file permissions. To correct this, please provide your username or domain name, and provide the location of the folder or files that need to have your ownership.

[RCT] All files and folders appear to be owned by me.  I was able to change permissions on all files and folders I tested.


3. Your .htaccess file has php_values or php_flags in it. This causes a 500 Internal server error when attempting to execute the script.

[RCT] None of the .htaccess files had any php_values or php_flags that I could see. 

In the home directory of the wiki (/public_html/wiki/), where index.php is found, the contents of .htacess are:

     AddHandler application/x-httpd-php5 .php

In the subdirectories, the contents of .htacess are:

      Deny from all
 
Is this a problem???

When I tried out MediaWiki, I ran into a small hurdle: it wants the PHP session directory to be writable, which it’s not since /etc/php.ini sets save_path to /var/lib/php/session which is writable by group apache but not by random user IDs.  I can configure PHP’s session.save_path in /etc/php.ini to point to a world-writable sticky directory like /tmp.  The planned default umask (072; all users share the same group) would work to protect session data from other users, I think.  But it feels unsafe; I’d rather have a separate session directory for each user.  I don’t think I can do that in a single global php.ini; the documentation and source code don’t reveal any kind of substitution going on in session.save_path which I could use to put in the current user’s home directory or some such.