MCrypt is used in PHP

Yesterday, when I decode the generated JSON file, I suddenly found that the return was null, (the JSON string in the JSON file was encrypted), and after repeated testing, it was found that the decode operation returned NULL as long as it contained the Chinese character in JSON and the decryption operation.        If no Chinese characters are normal decode.  Just start to think that is the encoding, repeated testing, or there are problems, and finally found the decryption operation inside, found the Mcrypt_decrypt function, search manual found that this function may be returned after the characters with hidden characters, will this cause?  Then use Str_replace ("" "," ", $json); The result came out, I rub, wasted half a day.