If your not getting the page by typing in
http://localhost/ check the httpd.config file. (don't forget the trailing " / ", that is what directs you further into the file)
In the httpd.conf, is the default address the browser will use to access your files. It is actually something like
http://localhost/xampp/index.php in the address bar. You shouldn't have to type all that in though.
on line 176 to 180 of the httpd.conf file:
# DocumentRoot: The directory out of which you will serve your
# documents. By default, all requests are taken from this directory, but
# symbolic links and aliases may be used to point to other locations.
#
DocumentRoot "/Applications/xampp/xamppfiles/htdocs"
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As for "http://localhost/administrator/", there is no such directory thus you should receive an error message.
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edit; and since your typing in //localhost/index.php and not getting that file is because the actual php file is not located in that directory. The html file in that directory is actually linking to the php in another directory.
I hope all this makes sence.