Many problems are more evident when the code is clean. Your code is a mess. It's hard for me to tell what's going on in there, and I'm very familiar with the inner workings of Wikis.

• Use meaningful variable names. $p and $q are terse, and you're overloading them. It's hard to follow what they're holding.
• Your life would be simpler if you would use a fixed name for the <textarea>, rather than creating one on the fly, and then hoping you can retrieve it given info plucked from the URL.
• Your life would be a lot simple if you would sort out which command is being invoked (e.g., edit, search) at the top, and call different top-level routines depending on the command. Your logic has a bit of a pretzel quality to it.
• Encode the pattern for matching a valid page name once, rather than sprinkling it throughout the code. The more duplication you have, the greater the chance that you'll get one subtley wrong.