php语法如何将数组转成JSON格式

//一个JSON字符串
$json = '{"name":"小米","age":30,"sex":"男","school":"清北"}';
  
//将JSON字符串转换成数组并保留key
$arr = json_decode($json, true);
  
//输出数组
print_r($arr);

Array
(
  [name] => 小米
  [age] => 30
  [sex] => 男
  [school] => 清北
)