How to convert htmlentities to UTF-8

Problem of converting UTF-8 from htmlentities to Chinese $ str = "Chinese"; echo & nbsp; json_encode ($ str); displayed as: [null] & nbsp; so I plan to use htmlentities & nbsp; please refer to the following link for details: $ str = htmlentities ($ str, UTF-8); echo & nbsp; j: converting htmlentities to UTF-8 in Chinese
$ Str = "Chinese ";
Echo json_encode ($ str );

Shown:
[Null]

So I want to use htmlentities to turn it over:
$ Str = htmlentities ($ str, UTF-8 );
Echo json_encode ($ str );

The result is a very depressing problem.
Notice: Use of undefined constant utf-assumed 'utf' in F: \ work \ test. php on line 24

["\ Ufffd \ u052a \ ufffd \ Alibaba \ ufffd \ uccef \ Alibaba \ ufffd \ u3d22 \ ufffd"


Line 24: $ str = htmlentities ($ str, UTF-8 );

The result is successfully converted, but I don't know why the error is reported!
------ Solution --------------------
The parameters must be enclosed in quotation marks. why not use iconv?