Sort Array From Database by Specific Key php

Question

I have the following array called posts which consists of data from my database:

Array ( [post_id] => 29 [user_id] => 2 [post_image_url] => images/w8y46ot7bsr3cpxhunq9lg2dkmf05jzvei1a.jpg ) 1
Array ( [post_id] => 41 [user_id] => 2 [post_image_url] => images/czmhng9j7qr0ite4d2w6obvl3p1k5yasxuf8.jpg ) 1
Array ( [post_id] => 42 [user_id] => 2 [post_image_url] => images/rk5e7yc9xolsd4mbnp6t2w0zaquf8i1j3ghv.jpg ) 1
Array ( [post_id] => 39 [user_id] => 2 [post_image_url] => images/wuvhtcyksmza70641n3xb5l2rqgdp8jefoi9.jpg ) 1
Array ( [post_id] => 40 [user_id] => 14 [post_image_url] => images/41io86ln7f9gcz3ep2dmvrjbya5k0wsxqthu.jpg ) 1

I am trying to sort it from greatest to least by "post_id." Currently I am using the following code:

usort($posts, function($a, $b) { 
    return $a->post_id > $b->post_id ? -1 : 1;
});      
foreach($posts as $i){
     echo print_r($i). "<br>";
}

However, this is not working. Any help would be appreciated.

How is it "not working" ? (Also you access array elements with [], not ->; Add error reporting to get useful error messages) — Rizier123
– Rizier123, Commented May 7, 2016 at 20:41
@Rizier123 it is "not working" because the values are not sorted in descending order by "post_id." If you look at the array, you can see the order of post_id is all over the place and not in the correct order — John55
– John55, Commented May 7, 2016 at 20:44
@splash58 That will not make a difference because these values are retrieved based on information in another array. Doing that makes no difference. — John55
– John55, Commented May 7, 2016 at 20:47

Ryan Tobin · Accepted Answer · 2016-05-07 20:52:46Z

1

Your problem is that you are accessing the element of the array with -> as opposed to []. Change this:

usort($posts, function($a, $b) { 
    return $a->post_id > $b->post_id ? -1 : 1;
});

To this:

usort($posts, function($a, $b) { 
    return $a[post_id] > $b[post_id] ? -1 : 1;
});

answered May 7, 2016 at 20:52

Ryan Tobin

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Comments

mickmackusa · Accepted Answer · 2017-08-16 23:24:52Z

Ryan's "correction" regarding accessing the array elements "means well", but it has syntax errors and will spew a bunch of these:

Notice: Use of undefined constant post_id - assumed 'post_id'

Furthermore his answer is more complicated than it needs to be. Because you are sorting on the first column in your subarrays, you can just use rsort() and avoid a user-defined/custom sort method. See for yourself:

Code: (Demo)

$posts=[
    ['post_id'=>29,'user_id'=>2,'post_image_url'=>'images/w8y46ot7bsr3cpxhunq9lg2dkmf05jzvei1a.jpg'],
    ['post_id'=>41,'user_id'=>2,'post_image_url'=>'images/czmhng9j7qr0ite4d2w6obvl3p1k5yasxuf8.jpg'],
    ['post_id'=>42,'user_id'=>2,'post_image_url'=>'images/rk5e7yc9xolsd4mbnp6t2w0zaquf8i1j3ghv.jpg'],
    ['post_id'=>39,'user_id'=>2,'post_image_url'=>'images/wuvhtcyksmza70641n3xb5l2rqgdp8jefoi9.jpg'],
    ['post_id'=>40,'user_id'=>14,'post_image_url'=>'images/41io86ln7f9gcz3ep2dmvrjbya5k0wsxqthu.jpg']
];
rsort($posts);

var_export($posts);

Output:

array (
  0 => 
  array (
    'post_id' => 42,
    'user_id' => 2,
    'post_image_url' => 'images/rk5e7yc9xolsd4mbnp6t2w0zaquf8i1j3ghv.jpg',
  ),
  1 => 
  array (
    'post_id' => 41,
    'user_id' => 2,
    'post_image_url' => 'images/czmhng9j7qr0ite4d2w6obvl3p1k5yasxuf8.jpg',
  ),
  2 => 
  array (
    'post_id' => 40,
    'user_id' => 14,
    'post_image_url' => 'images/41io86ln7f9gcz3ep2dmvrjbya5k0wsxqthu.jpg',
  ),
  3 => 
  array (
    'post_id' => 39,
    'user_id' => 2,
    'post_image_url' => 'images/wuvhtcyksmza70641n3xb5l2rqgdp8jefoi9.jpg',
  ),
  4 => 
  array (
    'post_id' => 29,
    'user_id' => 2,
    'post_image_url' => 'images/w8y46ot7bsr3cpxhunq9lg2dkmf05jzvei1a.jpg',
  ),
)

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