Hi,
We are happy to announce that XAMPP provides full 64-bit support for Windows since versions 5.6.40 / 7.1.26 / 7.2.15 / 7.3.2.
Previous versions of XAMPP includes 32bit binaries that are also compatible with Windows 64bit but there are features you can only use if you use the 64bit binaries E.g. PHP support native 64-bit integers, LFS, 64-bit memory_limit and much more.
This one of the most requested features for XAMPP for Windows, I hope this is useful!
Please post if you find any issues or you have any suggestions for future versions.