Do not edit any of the .php files that MediaWiki ships with. If you want to edit the content of your wiki pages, make use of the "Edit" tab while logged into your wiki.
If you want to implement custom code, write or install an extension. In any case, never edit the core .php files -- doing so will make future upgrades incredibly difficult as any custom modifications to core .php files are completely lost on upgrades and need to be reapplied (unlike extensions, which stick around). Furthermore, the core code changes in structure a lot between versions, and unless you're keeping up with MediaWiki development, your custom hack that works in one version may not work in the next one without significant rewrites.