PHP多维数组按日期排序

Question

I have a multidimensional array which contains data that has date fields. Some indexes may have single date fields, others may have multiple. An example of an index with multiple date is listed below.

Array ( [0] => Array ( 
    [0] => Array ()
... ... ... 
    [21] => Array ( 
       [0] => Array ( 
          [Date] => 2011-05-12
          [Color] => green
          [State] => Ohio
        )
       [1] => Array ( 
         [Date] => 1999-01-23
         [Color] => red
         [State] => Vermont
        )
       [2] => Array ( 
         [Date] => 3001-08-24
         [Color] => yellow
         [State] => Alaska
        )

I am trying to list them as the most current date to be the first entry down to the oldest entry. For example

 Array ( [0] => Array ( 
    [0] => Array ()
... ... ... 
    [21] => Array (
       [0] => Array ( 
         [Date] => 3001-08-24
         [Color] => yellow
         [State] => Alaska
        )
        [1] => Array ( 
          [Date] => 2011-05-12
          [Color] => green
          [State] => Ohio
        )
        [2] => Array ( 
         [Date] => 1999-01-23
         [Color] => red
         [State] => Vermont
        )

I have tried

function date_compare($a, $b){
    $t1 = strtotime($a["Date"]);
    $t2 = strtotime($b["Date"]);
    return $t2 - $t1;
}

I get an error of

Notice: Undefined index: Date in date_compare() for both lines of $t1 and $t2

And when I put

function date_compare($a, $b){
    $t1 = strtotime($a[0]["Date"]);
    $t2 = strtotime($b[0]["Date"]);
    return $t2 - $t1;
}

I get an error of

Notice: Undefined offset: 0 in date_compare(). On the second line $t2.

Note: The Array starts off with a [0] index, then it goes into [0], [1], [2].

Answer 1

First loop the array and sort the sub array using usort. For example considering the array to be

$array = array(array(
            array('Date' => '1999-01-23','Color' => 'red','State' => 'Vermont'),
            array('Date' => '3001-08-24','Color' => 'yellow','State' => 'Alaska'),
            array('Date' => '2011-05-12','Color' => 'green','State' => 'Ohio'),
            ),
            array(
            array('Date' => '2017-01-23','Color' => 'red','State' => 'Vermont'),
            array('Date' => '2017-08-24','Color' => 'yellow','State' => 'Alaska'),
            array('Date' => '2000-05-12','Color' => 'green','State' => 'Ohio'),
            )
       );

Sorting code:

function date_compare($a, $b)
{
    $t1 = strtotime($a['Date']);
    $t2 = strtotime($b['Date']);
    return $t2 - $t1;  // descending
}  
  $sorted_array = array();
// loop the array
foreach($array as $key=>$value){
    usort($value, 'date_compare'); // sort the array
    $sorted_array[$key] = $value;  // assign sorted array to new array
}

print_r($sorted_array);

Output

Array
(
    [0] => Array
        (
            [0] => Array
                (
                    [Date] => 3001-08-24
                    [Color] => yellow
                    [State] => Alaska
                )

            [1] => Array
                (
                    [Date] => 2011-05-12
                    [Color] => green
                    [State] => Ohio
                )

            [2] => Array
                (
                    [Date] => 1999-01-23
                    [Color] => red
                    [State] => Vermont
                )

        )

    [1] => Array
        (
            [0] => Array
                (
                    [Date] => 2017-08-24
                    [Color] => yellow
                    [State] => Alaska
                )

            [1] => Array
                (
                    [Date] => 2017-01-23
                    [Color] => red
                    [State] => Vermont
                )

            [2] => Array
                (
                    [Date] => 200-05-12
                    [Color] => green
                    [State] => Ohio
                )

        )

)

http://sandbox.onlinephpfunctions.com/code/631c4b904d937ad181ccff20cbd3fa89c697f06b