You cannot install XAMPP on Windows 10 IoT Core, because
XAMPP is compiled for Intel/AMD (x86) CPUs. The Raspberry Pi has an ARM CPU which is completely incompatible.
Windows 10 IoT Core is a minimal version of Windows, lots of stuff is missing. This includes the Desktop (explorer.exe, dwm.exe), support for Windows Forms etc. This means even a recompiled version of XAMPP might fail to run.