$_POST accepts NULL on AJAX request and prevents using PDO, how to fix?

Question

PHP

There is a form

<form id="filter">
    <input type="checkbox" name="day[]" value="1"> 
    <input type="checkbox" name="day[]" value="2">
    <input type="checkbox" name="day[]" value="3"> 
</form>

Values ​​are passed using AJAX

var a = 0;
var b = 5;
var c = $( "#filter" ).serialize();
var Data = 'a=' + a + '&' + 'b=' + b + '&' + c;
var process = false;
                    $.ajax({
                        url: 'script.php',
                        method: 'POST',
                        data: Data,
                        beforeSend: function() {
                            process = true;
                        }
                    });

In script.php var_dump($_POST['day']) produces NULL , var_dump($b); shows string(0) ""

$a = $_POST['a'];
$b = $_POST['b'];
$c = implode(',', $_POST['day']); // Warning: implode(): Invalid arguments passed
$in  = str_repeat('?,', count($c) - 1) . '?'; // Warning: Second argument has to be greater than or equal to 0
$sql = "SELECT * FROM table WHERE day IN ($in) ASC LIMIT :a, :b";
$stmt = $pdo->prepare($sql);
$stmt->execute($с, [$a, $b]);
$data = $stmt->fetchAll();
echo json_encode($data);

This doesn't help:

if (isset($_POST['day'])) {
    $c = implode(',', $_POST['day']);
}

ADDED: If you do not use prepared queries and pdo, but mysqli, then everything works, despite Warning: implode(): Invalid arguments passed

Answer 1

If you have NOT checked a single check-box, then you will not receive anything.
Those. if you submit the form from my example, $_POST will be:

array(2) {
  ["a"]=>
  string(1) "0"
  ["b"]=>
  string(1) "5"
}

Those. you don't have any day.
As I understand it, you are outputting data based on the value of this day. And that means nothing should come out.
That is, your code will boil down to:

$result = [];

$a = (int) $_POST['a'];
$b = (int) $_POST['b'];

$c = array_filter( (array) $_POST['day'] );

if ( !empty($c) )
{
  $c = implode(',', $_POST['day']);
  $in = str_repeat('?,', count($c) - 1) . '?';
  $sql = "SELECT * FROM table WHERE day IN ($in) ASC LIMIT :a, :b";
  $stmt = $pdo->prepare($sql);
  $stmt->execute($с, [$a, $b]);
  $data = $stmt->fetchAll();

  $result = $data;
}

echo json_encode($result);

PS Do you really need manual assembly of values? Why not put it in the form itself?

<form id="filter">
  <input type='hidden' name='a' value='0' />
  <input type='hidden' name='b' value='5' />
  <input type="checkbox" name="day[]" value="1"> 
  <input type="checkbox" name="day[]" value="2">
  <input type="checkbox" name="day[]" value="3"> 
</form>

$.ajax({
    url: 'index.php',
    method: 'POST',
    data: $("#filter").serialize()
});

Answer 2

$c = implode(',', $_POST['day']); // Warning: implode(): Invalid arguments passed

Change to
array_filter()