{"a":"\u00e1"} and {"a":"รก"} are different ways to write the same JSON document; The JSON decoder will decode the unicode escape.
In php 5.4+, php’s json_encode does have the JSON_UNESCAPED_UNICODE option for plain output. On older php versions, you can roll out your own JSON encoder that does not encode non-ASCII characters, or use Pear’s JSON encoder and remove line 349 to 433.