I am a PHP developer and love minimalistic, one file projects. I love wikis too.
My comments here is not intended to be negative, just some feedback, so you can improve the program (and maybe fix some things).
I tried to bring your wiki online at my localhost. Doesn't worked because you are using short open tags. Since many PHP installations have this Off by default, I think you should use the non-short open tag "<?php" instead just "<?".
I changed it but faced another issue. The Syntax <?php=$variable?> was giving me some render issues. I don't dive in to find why, but I changed it to <?php echo $variable; ?>
It worked! Well, so-so.
A page with a textarea rendered. I typed something and clicked on save, but the page reloaded and nothing happened. Tried filling the password too. When I click on "Wiki" link, nothing happens too.
If you need some more info to help you find why the app is having this behaviour, just tell me.