Sort array by highest key=>value

Question

I want to sort an array highest to lowest by the value of amount. My array $res is as follows:

Array
(
    [0] => 1
    [id] => 1
    [1] => Testowy 1
    [user] => Testowy 1
    [2] => 150
    [amount] => 150
    [3] => 1,2,3
    [what] => 1,2,3
    [4] => Polska
    [country] => Polska
    [5] => 1
    [platform] => 1
)
Array
(
    [0] => 2
    [id] => 2
    [1] => Testowy 2
    [user] => Testowy 2
    [2] => 100
    [amount] => 100
    [3] => 1
    [what] => 1
    [4] => United States
    [country] => United States
    [5] => 2
    [platform] => 2
)

I tried using max and arsort, but none of those seem to accept which key they should use for sorting. Any help?

Answer 1

usort($res, function ($a, $b){
    return $b['amount'] - $a['amount'];
});
print_r($res);

For versions of PHP < 5.3, use the following:

function cmp($a, $b){
    return $b['amount'] - $a['amount'];
}
usort($res, "cmp");

Answer 2

Try usort

function cmp($a, $b)
{
    return ($a["amount"]<=$b["amount"])?-1:1;
}

usort($array, "cmp");

Answer 3

Use sorting functions with user defined comparators, like: usort:

http://php.net/usort

Then your comparator gets two objects and tells (by any logic you want) which one is greater):

function compare($a, $b) {
    $result = -1;
    if( $a["amount"] == $b["amount"]) {
      $result = 0;
    } else {
       if( $a["amount"] > $b["amount"] ) {
          $result = 1;
       }
    }

    return $result;
}

usort($res, "compare");

Answer 4

Here is my solution, I know it is very late please don't mind

$data = [
    [
        'id' => 1,
        'user' => 'Testowy 1',
        'amount' => 100,
        'what' => '1,2,3',
        'country' => 'Polska',
        'platform' => 1,
    ],
    [
        'id' => 2,
        'user' => 'Testowy 2',
        'amount' => 130,
        'what' => '1',
        'country' => 'United States',
        'platform' => 2,
    ],
    [
        'id' => 3,
        'user' => 'Testowy 3',
        'amount' => 180,
        'what' => '1',
        'country' => 'United States',
        'platform' => 2,
    ],
];

usort($data, function ($a, $b) {
    $aValue = $a['amount'];
    $bValue = $b['amount'];

    // Sort in descending order
    return $bValue <=> $aValue;
});

echo "<pre>";
print_r($data);