sorting - questions on sort array by time in php

Question

---array $points----

Array
    (
        [0] => Array
            (
                [0] => 2011-10-02 05:30:00
                [1] => 20
            )

        [1] => Array
            (
                [0] => 2011-10-04 09:30:00
                [1] => 12
            )

        [2] => Array
            (
                [0] => 2011-10-01 13:30:00
                [1] => 25
            )

        [3] => Array
            (
                [0] => 2011-10-03 02:30:00
                [1] => 31
            )

    )

I have an array at above and would like to sort this array by time. Then I used the code as following to sort and result is correct. However, if I changed the code time[$key] = $val[0] to $time = $val[0], the result is wrong.

Is there anyone can explain this to me? Many thanks!

foreach($points as $key=>$val){

        $time[$key] = $val[0];

        array_multisort($time, SORT_ASC, $points);
    }

Answer 1

array_multisort sorts more than one array at once. However, it works on an array of columns, so the foreach loop is needed to get a column of the times. After building up this list, you can then perform the multisort. The $points array is ordered according to the indices in $times, as per this example in the docs.

However, you don't need to perform the sort inside the foreach, as that means the sort happens 4 times (in your example). It only needs to happen once:

foreach ($points as $key => $val) {
    $time[$key] = $val[0];
}

array_multisort($time, SORT_ASC, $points);