read key value json php

1// json object.
2    $contents = '{"firstName":"John", "lastName":"Doe"}';
3
4    // Option 1: through the use of an array.
5    $jsonArray = json_decode($contents,true);
6
7    $key = "firstName";
8
9    $firstName = $jsonArray[$key];
10
11
12    // Option 2: through the use of an object.
13    $jsonObj = json_decode($contents);
14
15    $firstName = $jsonObj->$key;
16