http://example.com/index.php works, but http://example.com/ gives a ‘404 Not Found’ error
ANSWER:
Uncomment (remove the ”#” from) the following line in your .htaccess file:
#DirectoryIndex index.php index.html
If that doesn’t help, or causes a 500 Internal Server error, contact your web host tech support and ask them about enabling DirectoryIndex for PHP files.