Usort on Array of DateTime Objects in PHP - No Error, Array Not Sorting

Question

I have a long multi dimensional array including some DateTime objects. I want to sort the entire array based on the [startdate] object. I'm using this usort function (based on a lot of online resources and modifications from various errors). It's not sorting the array.

My array is formatted as follows. Showing only the start for brevity.

Array
(
    [] => Array
        (
            [startdate] => DateTime Object
                (
                    [date] => 2019-04-10 19:00:00.000000
                    [timezone_type] => 3
                    [timezone] => America/New_York
                )

            [starttime] => DateTime Object
                (
                    [date] => 2019-04-10 19:00:00.000000
                    [timezone_type] => 3
                    [timezone] => America/New_York
                )

Here is my usort code:

function date_compare($a, $b) {
    $t1 = $a["startdate"]->format('Y-m-d H:i:s');
    $t2 = $b["startdate"]->format('Y-m-d H:i:s');           
    return $t1 - $t2;
}    
usort($allEventsSimple, 'date_compare');

I also tried sorting the DateTime objects using this code:

function date_compare($a, $b) {
    $t1 = var_dump($a[startdate]);
    $t2 = var_dump($b[startdate]);          
    return $t1 - $t2;
}    
usort($allEventsSimple, 'date_compare');

Just do strtotime($a["date"]) - strtotime($b["date"]). You will also need to loop over the elements of the base array. — Aniket Sahrawat
– Aniket Sahrawat, Commented Apr 5, 2019 at 3:14
Missed that, try strtotime($a['startdate']->date) - strtotime($b['startdate']->date) @AmyMcGarity-GermanPearls — Aniket Sahrawat
– Aniket Sahrawat, Commented Apr 5, 2019 at 3:21
@Beginner I can't see getTimestamp in the dump, it might not work. — user7780894
– user7780894, Commented Apr 5, 2019 at 3:23

Phil · Accepted Answer · 2019-04-05 03:21:11Z

2

You actually do have errors, you just can't see them (see How to get useful error messages in PHP?)

Your code would be producing

PHP Notice: A non well formed numeric value encountered in ...

This is because you're attempting to subtract one non-numeric string from another.

If you're using PHP 7, you can directly compare DateTime instances with the awesomely named "spaceship operator", eg

return $a['startdate'] <=> $b['startdate'];

If you're stuck on PHP 5, compare timestamps

return $a['startdate']->getTimestamp() - $b['startdate']->getTimestamp();

edited Apr 5, 2019 at 3:21

answered Apr 5, 2019 at 3:09

Phil

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4 Comments

Amy Over a year ago

No, I'm on php 5.6

Phil Over a year ago

@AmyMcGarity-GermanPearls that's why I provided a PHP 5 version

Amy Over a year ago

Thank you. I didn't see it - nothing past the image loaded.

Phil Over a year ago

@AmyMcGarity-GermanPearls can you see it now? If not, try reloading this page

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