How to sort multidimensional array using value of 2 specific keys

You have a few of issues here. First, you only want to sort the products array within the results, so pass that into your usort functions instead of the entire results array.

Second, you need to make your usorts smarter. They are just functions that return an indication of how the elements compare, you can put whatever logic you want inside them.

The third thing I see is that you do not seem to want the results sorted by brand name alphabetically, you want Samsung first. I'm guessing you will want to define a custom sort order for brands that put the most popular brands first. Or maybe you work for a brand that always wants to be listed first, or have promotional agreements - whatever.

Here are two ways you can solve the problem. The first is a simple usort function that compares based upon the brand name alphabetically, and the display order in the results. The second uses a basic class to define a custom sort order, and a compare function that uses that list, and falls through to the basic comparisons if the brands are the same, or not present in the list. This shows how you can use classes in usort, which some people seem to struggle with.

<?php
$result = [
    'products' => [
        [
            'LOTid'        => [20200901000001],
            'Brand'        => 'SAMSUNG',
            'Product'      => 'TV',
            'DisplayOrder' => 6
        ],
        [
            'LOTid'        => [20200901000002],
            'Brand'        => 'LG',
            'Product'      => 'TV',
            'DisplayOrder' => 9],
        [
            'LOTid'        => [20200901000003],
            'Brand'        => 'SAMSUNG',
            'Product'      => 'MOBILE',
            'DisplayOrder' => 1],
        [
            'LOTid'        => [20200901000002],
            'Brand'        => 'LG',
            'Product'      => 'MOBILE',
            'DisplayOrder' => 4]
    ],
    'status'   => 'ok'
];

// Simple sort based upon contents of the results
function sortByBrandAndDisplayOrder($a, $b)
{
    // Sort on brand name
    if($a['Brand'] < $b['Brand'])
    {
        return -1;
    }
    if($a['Brand'] > $b['Brand'])
    {
        return 1;
    }

    // Sort on display order if brands are equal
    return $a['DisplayOrder'] <=> $b['DisplayOrder'];
}

//Pass in the products section of the results - you don't want to sort the outer results array
usort($result['products'], 'sortByBrandAndDisplayOrder');
print_r($result);

// Make a class that can hold a custom order definition
class ProductSort
{
    private static $_brandOrder = [
        'SAMSUNG'=>0,
        'LG'=>1
    ];

    static function sortByBrandAndDisplayOrder($a, $b)
    {
        //If both brands are present, sort on the custom brand order
        if(array_key_exists($a['Brand'], self::$_brandOrder) && array_key_exists($b['Brand'], self::$_brandOrder))
        {
            // If both brands have the same display order (same brand), will fall through to
            if(self::$_brandOrder[$a['Brand']] < self::$_brandOrder[$b['Brand']])
            {
                return -1;
            }
            if(self::$_brandOrder[$a['Brand']] > self::$_brandOrder[$b['Brand']])
            {
                return 1;
            }
        }
        elseif(array_key_exists($a['Brand'], self::$_brandOrder))
        {
            return -1;
        }
        elseif(array_key_exists($b['Brand'], self::$_brandOrder))
        {
            return 1;
        }

        // Sort on brand name
        if($a['Brand'] < $b['Brand'])
        {
            return -1;
        }
        if($a['Brand'] > $b['Brand'])
        {
            return 1;
        }

        //Sort on DisplayOrder in the results
        return $a['DisplayOrder'] <=> $b['DisplayOrder'];
    }
}

usort($result['products'], ['ProductSort', 'sortByBrandAndDisplayOrder']);
print_r($result);