// json object.
    $contents = '{"firstName":"John", "lastName":"Doe"}';

    // Option 1: through the use of an array.
    $jsonArray = json_decode($contents,true);

    $key = "firstName";

    $firstName = $jsonArray[$key];


    // Option 2: through the use of an object.
    $jsonObj = json_decode($contents);

    $firstName = $jsonObj->$key;