How can I use prepared statements combined with Transactions with PHP?

Question

My goal is to use a transaction and a prepared statement simultaneously, to achieve both integrity of data, and prevention of SQL injection.

I have this:

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Answer 1

try 
{
    $cnx = new PDO ($dsn,$dbuser,$dbpass);   
    $cnx->setAttribute (PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

    $cnx->beginTransaction ();

    $stmt = $cnx->prepare ("SELECT * FROM users WHERE username=?");
    $stmt->execute(array($username));

    $cnx->commit();

    while ($row = $stmt->fetch (PDO::FETCH_OBJ)){
        echo $row->userid;
    }
}

catch (Exception $e) { 
    if (isset ($cnx)) 
        $cnx->rollback ();
       echo "Error:  " . $e; 
    }
}