I would like to know how you upgrade PHP in Xampp for Windows? I tried to download the latest PHP version from the main PHP site but when I check (phpinfo) I still get that the previous version is still in use.
Take a backup of your htdocs and data folder (subfolder of MySQL folder), reinstall upgraded version and replace those folders.
Note: In case you have changed config files like PHP (php.ini), Apache (httpd.conf) or any other, please take back up of those files as well and replace them with newly installed version.
You can go through the following link as it helped me, should work for you as well. http://hype-free.blogspot.com/2007/07/updating-php-in-xampp-for-windows.html
Realizing that my answer helped couple of users, here is the edit from original link:
NOTE: there are two directories to be updated with new version of files, namely
php sub-directory and
apache/bin sub-directory, inside XAMPP installation.