内部功能没有选择数据库，PDO的解决方案是什么？

Question

I'm attempting to perform a query inside a function but it return me "No database selected". I regularly have re-initialized the PDO object inside it

$db = "mysql:host=localhost;dbname=my_database";
$pdo = new PDO($db, 'dbname', 'dbpassword');
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
//some operations.. Works fine!

function sendMail($to)
    {
      $db = "mysql:host=localhost;dbname=my_database";
      $pdo = new PDO($db, 'dbname', 'dbpassword');
      $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

      $qryString="SELECT Codice FROM users WHERE Mail=:mail";
      $qry = $pdo->prepare($qryString);
      $params = array("mail" => $to);
      $qry->execute($params); //won't work
    }

Note that operations on the DB outside the function works fine.

The problem is that the code won't work neither passing the global $pdo object. This is the actual code

 $db = "mysql:host=localhost;dbname=my_database";
 $pdo = new PDO($db, 'dbname', 'dbpassword');
 $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

 sendMail($mail, $pdo)

function sendMail($to, PDO $pdo)
        {
          $qryString="SELECT Codice FROM users WHERE Mail=:mail";
          $qry = $pdo->prepare($qryString);
          $params = array("mail" => $to);
          $qry->execute($params); 
        }

Answer 1

Pass the PDO object to the function, because the scope of $pdo is outside the scope of sendMail().

function sendMail(PDO $pdo, $to) {
    $queryString = "SELECT Codice FROM users WHERE Mail=:mail";
    $statement = $pdo->prepare($queryString);
    $params = array("mail" => $to);
    $result = $statement->execute($params);
}

or

function sendMail($to) {
    global $pdo;
    $queryString = "SELECT Codice FROM users WHERE Mail=:mail";
    $statement = $pdo->prepare($queryString);
    $params = array("mail" => $to);
    $result = $statement->execute($params);
}