What exactly do you mean by encrypt the page?
Encrypt the source so some one cannot view its content when opening the file?
Encrypt the output? What?...
If you want to encrypt the raw source files then when they run on the server they'll need to be something that mod_php can read. So you'll have to encrypt them, when you get a request decrypt them then pass to php to run. Not likely siomething that you'd want to do.... Not anything I've tried either.
I guess it would be possible manually crypt (or write a program that will) then write a C apache module that will decrypt these files upon request and send to mod_php to run. You'll need access to the server's httpd.conf file and create a new mime type and AddType declaration for your new module (I'm not sure if these options can be done from a .htaccess file). This would be very time consuming and put more strain on the server.