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$result = [];

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

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

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

echo json_encode($result);