I have this array :

```
Array
(
    [0] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => C4
            [price] => 53.83600000
        )

    [1] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => RE
            [price] => 53.83600000
        )

    [2] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => C4
            [price] => 0.09188900
        )

    [3] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => RE
            [price] => 0.09188900
        )

    [4] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => C4
            [price] => 584.80000000
        )

)

```

and I'm expecting result like this (sort by robo DESC) :

```
Array
(
    [0] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => RE
            [price] => 53.83600000
        )

    [1] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => RE
            [price] => 0.09188900
        )

    [2] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => C4
            [price] => 53.83600000
        )

    [3] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => C4
            [price] => 0.09188900
        )

    [4] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => C4
            [price] => 584.80000000
        )

)

```

and I already done this : 

```
    usort($dc_array_process, function($a, $b) {
        return $a['robo'] - $b['robo'];
    });

```

but my array still not in DESC order. any idea what did I do wrong?