Problem json_encode utf-8

I have a problem with json_encode function with special characters.

For example I try this:

$string="Svrček";

echo "ENCODING=".mb_detect_encoding($string); //ENCODING=UTF-8

echo "JSON=".json_encode($string); //JSON="Svr\u010dek"

What can I do to display the string correctly, so JSON="Svrček"?

Thank you very much.