How to get an array from PHP keys?

Question

PHP

From js to php, an array comes in the form of a string, because it is sent like this.

const a = [{3:5},{1:2}];
JSON.stringify(a)

I need an array of keys in php, i.e. To then make a request to the db
$arr = [3,1];

$goods = mysqli_query($connection, "SELECT * FROM `goods` WHERE `id` in $arr");

Yes, I know similar to the previous question, I just formatted the question incorrectly.

Answer 1

$str ='[{"3":5},{"1":2}]';
$arr = json_decode($str, true);
$result = array_map(
    function ($el) {
        return array_keys($el)[0];
    },
    $arr
);
var_dump($result);
// array(2) {
//  [0] => int(3)
//  [1] => int(1)
// }