I encountered the same issue when I converted to Xampp from the builtin WEB server on Mac OSX.
Here is how to resolve the issue
In applications, open the Xampp folder
In the Xampp folder, launch Manager-OSX
Click on the Manage Servers tab
Highlight Apache Web Server
Click on the Configure button
Click on the Open Conf File button (this will open the httpd.conf file in Text Editor for you)
Click on the YES button when prompted
In the httpd.conf file, scroll down to the section:
DirectoryIndex index.php index.html index.html.var index.php3 index.php4
change the first entry from index.php to what you need it to be and then save the changed httpd.conf file
Click the OK button in the Configure Apache WEB server window
WIth Apache WEB server still highlighted, click on the Restart button
Now when you open localhost on your WEB server, it should open to where you need it to open to.