Pop one integer, use it as a multi-byte string character code and push the newly created string on the stack.
The integer can be any number from 0 to 65535, although many codes are not valid characters.
For more information about valid characters, please, check out the Unicode Consortium.
If you have not seen it yet, the W3C now has a new look.
They are using a Web 2.0 system with PHP running on their server along Apache version 2.x. In other words, they most certainly are running Linux as their server operating system.
And for those who do not know that yet, this is the consortium that takes care of making your browser work.