How can use ORDER BY in PDO query with PHP? And oversize memory if it find NULL field

Question

I have this PHP code, running on Ubuntu server 22.04:

$query = "SELECT * FROM info ORDER BY data DESC LIMIT 20";
$mdb_file = 'database.mdb';
$uname = explode(" ",php_uname());
$os = $uname[0];
switch ($os){
  case 'Windows':
    $driver = '{Microsoft Access Driver (*.mdb)}';
    break;
  case 'Linux':
    $driver = 'MDBTools';
    break;
  default:
    exit("Don't know about this OS");
}
$dataSourceName = "odbc:Driver=$driver;DBQ=$mdb_file;";
$connection = new PDO($dataSourceName);
$sth = $connection->prepare($query);
$sth->execute();
foreach ($sth as $row) {
  print_r($row);
}

but when it finds a null field, it returns oversized memory.

Other problem is that when I use ORDER BY, it doesn't work, it returns

Uncaught PDOException: SQLSTATE[]: <>: 1 Couldn't parse SQL

I've tried to remove the null fields and remove ORDER BY, and it works, but that's not what I want.