PHP7 UTF-8 codepoint escape sequence with variable

Question

PHP 7 introduced Unicode Codepoint Escape Syntax which is very easy to use, but I can't get it to work with a variable as a codepoint.

echo "\u{1f602}"; // echoes 

$em = "1f602";
echo sprintf("\u{%x}", $em); // returns error "Invalid UTF-8 codepoint escape sequence"

What am I doing wrong ?