json_decode mistery

Hello, I am usin json regulary, but in one script I have mistery: echo($decrypted_data)."\n\n"; var_dump(json_decode($decrypted_data, true)); echo "\n"; var_dump(json_decode('{"result_ok":true,"result_message":null,"client_name":"Radek Krej\u010da"}', true)); I got: {"result_ok":true,"result_message":null,"client_name":"Radek Krej\u010da"} NULL array(3) { ["result_ok"]=> bool(true) ["result_message"]=> NULL ["client_name"]=> a" } string(13) "Radek KrejÄ You can see, that in $decrypted_data, is stored valid (by my opinion) json data. If I use this variable in json_decode, I got "null". And if I manualy use data displayed on screen, I got valid array. Where I do mistake? If I remove client_name (so no utf8 is in json data), situation is the same. Radek