Well, I have a completely AJAX driven site. I inserted a jQuery code that affects all forms and queries site-wide. It's to the point, even though I want to change it, I fathom and accept the idea of a website that utitlizes a single function to process all queries (search, links, & profile, etc....)
How do you accomodate speed and security to such a platform? My php files can be accessed directly from their location's link. That is a threat. Help me; as well as AJAX, I need validation and '777' protection.
Before you read my answer read also this (as answer to a comment on your question) :
Possible to view PHP code of a website?
Don't put speed and security in the same box.
A website can be secure and fast at the same time.
I would secure a folder with 777 access (why not 755?) with an empty 0Kb index.html file (yes, even if you have inside a bunch of .php files!) ad put an .htaccess with restrictions deny from all that allows a folder to be accessed internally but not from 'outside'.
Than I would NEVER send sensitive data through the requests, but rather a client-side transformed Hashing algorithm like MD5 or SHA1 to compare data and validate on server-side. So don't ever ever send sensitive data in it's pure state over the yellow wire.
Need more security? https
Regarding a "single function" that drives your JS client-end of the site, well, if well formatted the browser doesn't care if it's one or hundreds, a good code is a readable code. Performance wise there's lots of suggestions on the www how to speed up your code.
To add to a really good @Tim's comment/tip, you can still open your console in Firebug (Net) and inspect every single piece of information that is sent from your Page to the server (and vice-versa!!) and act accordingly.