Communication between PHP and js (via ajax, json)_javascript skills

JavaScript side:
Note: xmlHttp.setRequestHeader must be set, otherwise the parameters passed to PHP will become null (line 38)
The highlight is line 31!

PHP [testJson.php] :
(Note that the php file must be clean, and there cannot be other tags outside the tag, otherwise the eval function cannot parse)
The highlight is line 6

Summary:
js needs to send data to PHP, using xmlHttp.send( "id=123");
PHP sends data to js, ​​using echo json_encode($res); (note that the construction of the variable $res should comply with JSON specifications)
js needs to parse the data sent by PHP For data in JSON format, use var obj = eval('(' xmlHttp.responseText ')');