How does the PHP function usort() work in sorting arrays, and when is it recommended to use it?

The PHP function usort() is used to sort an array by values using a user-defined comparison function. This function allows for custom sorting logic to be applied to the array elements. It is recommended to use usort() when you need to sort an array based on a specific criteria that is not covered by the built-in sorting functions in PHP.

// Example usage of usort() to sort an array of numbers in descending order
$numbers = [5, 2, 8, 1, 9];

usort($numbers, function($a, $b) {
    if ($a == $b) {
        return 0;
    }
    return ($a > $b) ? -1 : 1;
});

print_r($numbers);