Question

I have problem with sorting this array:

Array
(
    [0] => stdClass Object
        (
            [id] => 120
            [registration_date] => 2012-10-19 16:57:46
            [username] => Jeff
        )
    [1] => stdClass Object
        (
            [id] => 121
            [registration_date] => 2012-12-23 16:57:46
            [username] => Peter
        )
    [2] => stdClass Object
        (
            [id] => 122
            [registration_date] => 2012-11-30 16:57:46
            [username] => Susan
        )

)

It is stored in variable:

$unsorted_users

I want to order this array DESC by registration_date so it looks like this:

Array
    (
        [0] => stdClass Object
            (
                [id] => 121
                [registration_date] => 2012-12-23 16:57:46
                [username] => Peter

            )
        [1] => stdClass Object
            (
                [id] => 122
                [registration_date] => 2012-11-30 16:57:46
                [username] => Susan

            )
        [2] => stdClass Object
            (
                [id] => 120
                [registration_date] => 2012-10-19 16:57:46
                [username] => Jeff
            )

    )

and it is ordered by registration_date like:

2012-12-23 16:57:46
2012-11-30 16:57:46
2012-10-19 16:57:46

Instead of original where it was like:

2012-10-19 16:57:46
2012-12-23 16:57:46
2012-11-30 16:57:46

I am using this code, but it is not working good (the print_r($sorted_users); output of $sorted_users is “1”. I don’t why it is 1 and not sorted array.)

$sorted_users = usort($unsorted_users, function($a, $b) {
   return $a['registration_date'] - $b['registration_date'];
});

Any advice why is my code for $sorted_users wrong?

Answer 1

You can’t substract (that) strings. If you want to compare two strings, use the strcmp() function like this:

<?php

//  setup
$unsorted_users = array();

$t = new stdClass(); 
$t->id = 120; $t->username = 'jeff'; $t->registration_date = '2012-10-19 16:57:46'; 
$unsorted_users[] = $t;

$t = new stdClass(); 
$t->id = 121; $t->username = 'Peter'; $t->registration_date = '2012-12-23 16:57:46'; 
$unsorted_users[] = $t;

$t = new stdClass(); 
$t->id = 122; $t->username = 'Susan'; $t->registration_date = '2012-11-30 16:57:46'; 
$unsorted_users[] = $t;
// end setup


$sorted_users =    // this var is nor relevant as it shoud be allways true
usort($unsorted_users, function($a, $b) {
   return strcmp ($a->registration_date, $b->registration_date) ;
});

echo '<pre>';
print_r($unsorted_users)   // that is in fact ordered!

?>

fiddle here