How to calculate difference between values ​​in JSON?

Question

PHP

We have a data.json file with the following content

[
 ["11.09.2018","222"],
 ["12.09.2018","392"],
 ["13.09.2018","485"],
 ["14.09.2018","449"],
 ["15.09.2018","449"],
 ["16.09.2018","467"]
]

How to calculate difference between next and previous value? (subtract 222 from 392) and put the results in another file as:

[
 ["1","170"], //392-222
 ["2","93"], //485-392
И так далее

And all this in PHP...
But before that, directly get the value of the data.json file like this

$file = file_get_contents('data.json');
$list = json_decode($file,TRUE);

Answer 1

$data = json_decode($data);
$data = array_column($data, 1);
 
$result = [];
for($i = 1, $size = count($data); $i < $size; ++$i) {
  $result[$i] = $data[$i] - $data[$i - 1];
}

https://ideone.com/ZGf0iR