Sort Multi-dimension array by date with usort

Question

I'm trying to sort a multi-dimensional array by date using usort() but I can't seem to get it to work.

Start array:

$dayEvents = array();
$dayEvents['output'] = array();

And it is assigned like so:

$dayEvents['output'][] = array('date' => $dateStamp, 'data' => $dataOutput, 'ad' => $allDay);

Example output:

array(1) {
["output"]=>
  array(6) {
    [0]=>
    array(3) {
      ["date"]=>
      string(19) "2014-03-12 00:00:00"
      ["data"]=>
      string(115) "
2 tests were booked
"
      ["ad"]=>
      int(1)
    }
    [1]=>
    array(3) {
      ["date"]=>
      string(19) "2014-03-12 08:30:00"
      ["data"]=>
      string(316) "
08:30am
Matamata Class 1 R & F
"
      ["ad"]=>
      int(0)
}
    [2]=>
    array(3) {
      ["date"]=>
      string(19) "2014-03-12 08:00:00"
      ["data"]=>
      string(319) "
08:00am-04:00pm
Truck Course
"
      ["ad"]=>
      int(0)
    }
    [3]=>
    array(3) {
      ["date"]=>
      string(19) "2014-03-12 08:00:00"
      ["data"]=>
      string(328) "
08:00am-03:30pm
Trade Ed Rot Class 2
"
      ["ad"]=>
      int(0)
    }
    [4]=>
    array(3) {
      ["date"]=>
      string(19) "2014-03-12 08:00:00"
      ["data"]=>
      string(326) "
08:00am-03:30pm
Trade Ed Tga Class 2
"
      ["ad"]=>
      int(0)
    }
    [5]=>
    array(3) {
      ["date"]=>
      string(19) "2014-03-12 17:00:00"
      ["data"]=>
      string(330) "
05:00pm-08:00pm
Tauranga Truck Course
"
      ["ad"]=>
      int(0)
    }
}
}

The statement to sort it:

if(count($dayEvents['output'])>1) {
    uasort($dayEvents, 'date_compare');
}

And the function itself:

function date_compare($a, $b) {
    return strtotime($a['date']) > strtotime($b['date']);
}

And finally to output the data

foreach($dayEvents as $outputData) {
    $calendar .= $outputData['data'];
}

But it just isn't sorting by date, and I get the error Notice: Undefined index: data

Can anyone see where I'm going wrong? I've never used usort before and all the instructions I've tried to follow from questions on here don't seem to work.

Answer 1

I believe the problem is in your foreach statement. Instead of this:

foreach($dayEvents as $outputData) {

You need this:

foreach($dayEvents['output'] as $outputData) {

For the first statement, data is undefined, since you have a single element in that array called 'output'. In the second, 'data' should be defined.

You probably also need to add that into your uasort call:

uasort($dayEvents['output'], 'date_compare');

Answer 2

The array-sorting functions in PHP only work with 1-dimensional arrays. To sort 2D-arrays you can use the following function of mine that I always use for these kind of tasks:

function array_sort_by_column(&$array, $column, $direction = SORT_ASC) {
    $reference_array = array();

    foreach($array as $key => $row) {
        $reference_array[$key] = $row[$column];
    }

    array_multisort($reference_array, $direction, $array);
}

Adjusted your code:

array_sort_by_column($dayEvents['output'], 'date');