OK, that error message is normal and is working as it should.
What I want to do is quickly see if the issue is with your paths in the index.php of your own file.
So open the \xampp\apache\conf\extra\httpd.conf file in your text editor and look for this section and change it by adding the IP you are using to access the ip/mydirectory/index.php file
# New XAMPP security concept
Deny from all
Allow from 127.0.0.0/8
Allow from add the ip here
ErrorDocument 403 /error/HTTP_XAMPP_FORBIDDEN.html.var
Save the file and restart Apache.
Now try and access http://ip/xampp/index.php
and tell me if any of the links work on the page.
Make sure that the links on your own page are not prefixed with localhost or 127.0.0.1 as they mean they will only be available for the PC that XAMPP is installed on.
What is the index.php related to, is it a CMS or WordPress etc?