Why does XAMPP handle all Greek letters except one?

[MelerEcckmanLawler]

So I have a Greek numeral calculator (where Greek letters are used as numbers) written in PHP and it works fine when I host it on 000webhost but when I use localhost it shows lowercase Greek upsilon as a question mark. It shows the rest of the alphabet correctly though. How can I fix this so I can have consistent results between xampp and 000webhost?

This is lowercase Greek upsilon: υ