Why not install the XAMPP version that includes the php version you require then when all the script authors have finally caught up then simply install the lasted XAMPP version again?
Upgrading PHP has always had deprecated code issues from way back and it looks like it will continue into the foreseeable future.
Back versions of XAMPP can be obtained from the SourceForge Windows Archive
You can also download the appropriate version in a zip format and extract the files you require.
Alternatively contact the script's author and see if there is an upgrade to fix the deprecated code error message issue or are at least are contemplating a script update (or is there even an entry in the scripts configuration that will allow you to eliminate php error messages?
) - XAMPP has, in the php.ini file, disabled this error message in 1.7.2 error_reporting = E_ALL & ~E_NOTICE & ~E_DEPRECATED
I am not sure if it would be a good idea to simply use an older version php.ini on the latest version of php - others may be able to help with this observation.
This Advanced Forum Search
has some useful comment from one of XAMPP For Windows developers.