Unicode character in PHP string

This question looks embarrassingly simple, but I haven’t been able to find an answer.

What is the PHP equivalent to the following C# line of code?

string str = "\u1000";

This sample creates a string with a single Unicode character whose “Unicode numeric value” is 1000 in hexadecimal (4096 in decimal).

That is, in PHP, how can I create a string with a single Unicode character whose “Unicode numeric value” is known?