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:

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

        $cnx->beginTransaction();
        $cnx->query("SELECT * FROM users WHERE username=$escaped_input");
        $cnx->query("SELECT * FROM othertable WHERE some_column=$escaped_input_2");

        $cnx->commit();
    }

    catch (Exception $e){
           $cxn->rollback();
           echo "an error has occured";

    }

I would like to incorporate the query as one would with a prepared statement:

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

$stmt_2=$cxn->prepare("SELECT * FROM othertable WHERE some_column=?");
$stmt_2->execute(array($user_input_2));

How can I achieve that?

Edit

I get this error:

PHP Parse error: syntax error, unexpected T_CATCH

Here is my updated code:

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 $stmt->userid;

}

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

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; 
    }
}