Everything was going great until I added
AddHandler application/x-httpd-php5s .php to the .htaccess file in my local server's document root (which I change frequently depending on the site I'm working with). Since I did that when I visit http://localhost:8888 my browser just downloads the index.php and it's not processed at all, just the raw code. Now I removed that line from the .htaccess file but I'm still having this problem.
I've found that if I add an alternative entry to my hosts file for 127.0.0.1 the new entry behaves like 'localhost' used to. But if I add the line above to my .htaccess it knocks out that new host as well. I've tried reinstalling MAMP and clearing its caches and all the temporary files I could find. I surfed through Apache's httpd.conf file all to no avail.
So, to be clear: http://localhost:8888 is experiencing the above problem. If I add a new entry to my hosts file for 127.0.0.1, say 'goomba' and the above line is not in the root .htaccess (and has never been for that host/alias/whatever) then I can access http://goomba:8888 just fine. But if I do add that line to the .htaccess then I have to add yet another entry to my hosts file to get around it even if I remove that line from the the .htaccess file.
I'm fine with using a different 127.0.0.1 alias (host? what is that called?) but it's bugging me that this is still broken.
Just to be clear, I'm on Mac OS Leopard (but I'm not using the built in Apache setup, but MAMP).
You are applying a mimetype where a handler should be (see documentation on handlers)
Try this instead:
EDIT: As you have indicated caching modules are loaded, you could read up on caching and htcacheclean (to clear the disk cache). You can also temporarily use the CacheDisable directive. One other thing that you could also try is to rename the file that you have requested (e.g. index.php -> index.bak), request the file again in the browser (should now 404), then revert and try again.