UTF-8 to GBK encoding problems, what are the good solutions

UTF-8 to GBK encoding problems, what are the good solutions? Iconv must be added with the & nbsp; IGNORE & nbsp; parameter to complete the conversion. Otherwise, an error will be aborted, while & nbsp; mb_convert_encoding is a file that can complete the conversion but can be converted, sometimes Chinese comments are replaced ???. I developed the system is the UTF-8 version, when the release of the UTF-8 to GBK encoding problem, what is the good solution?
Iconv must be added with the IGNORE parameter to complete the conversion. Otherwise, an error will be terminated. while mb_convert_encoding can complete the conversion, but sometimes the converted file will replace the Chinese comments ???.
I developed the system is the UTF-8 version, the release of the need to convert a GBK version, there is no good solution, please help. Share:
------ Solution --------------------
Do you know which characters cannot be converted?
Now, can we solve this problem?

You may post a base64 encoded string containing non-convertible characters.