How do I sort a flat Json by Date?

Question

I am pretty new to php and Jsons and I am trying to arrange contents of a Json by date. I know about the usort() function but so far I have been unsuccessful at using it. This is the Json:

[{"id":"1","poi_id":"36","date":"1993-05-14","url":"#","p_flag":"0"},{"id":"2","poi_id":"36","date":"2000-05-14","url":"#","p_flag":"0"},{"id":"3","poi_id":"36","date":"1992-05-14","url":"#","p_flag":"0"}]

What I have been trying to do is this:

function sortByYear($a, $b) {
    $dA = new DateTime($a['date']);
    $dB = new DateTime($b['date']);

    return $dA->format('y') - $dB->format('y');
}
$data=json_decode($unsorted, true);
print_r(usort($data, 'sortByYear'));

Answer 1

<?php
    $unsorted = '[{"id":"1","poi_id":"36","date":"1993-05-14","url":"#","p_flag":"0"},{"id":"2","poi_id":"36","date":"2000-05-14","url":"#","p_flag":"0"},{"id":"3","poi_id":"36","date":"1992-05-14","url":"#","p_flag":"0"}]';

function sortByYear($a, $b) {
    $dA = new DateTime($a['date']);
    $dB = new DateTime($b['date']);

    return $dA > $dB;
}
$data=json_decode($unsorted, true);
usort($data, 'sortByYear');
print_r($data);

Demo

A few points:

1. You need to sort by the full year Y, not the last twp digits y. When you cross over the new millennium you have problems.

2. I used > for the comparison. It is clearer what the sort is that way.

3. usort() sorts in place so no array is returned. This means you need to call var_dump() on the original array, not usort().