Why I get Syntax error when I prepare statement for dropping a Db? [duplicate]

In my code I try to drop a database:

  $connectionString="mysql://...";
  $pdo = new PDO($connectionString);

  ...

  // I intentionally Drop the Db from the user-given db name
  // Details ommited for simplicity
  $testDbName=$_POST['db_name'];
  $stmt = $pdo->prepare("DROP DATABASE :db");
  $stmt->bindValue(":db", $testDbName);
  $stmt->execute();

But I get the following error:

SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ''test_php_app_1076'' at line 1.ESQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ''test_php_app_1076'' at line 1

But if I try this:

  $pdo->query("DROP DATABASE `" . str_replace('`', '``', $testDbName) . "`");

Seem to work fine. Is there a way to make it work using prepared statements?