Having trouble with PDO queries (Notice: Undefined index)

PHP Snippet 1:

$row = $resultBySQL->fetchAll(PDO::FETCH_ASSOC);

PHP Snippet 2:

//notice the 'fetch' in place of 'fetchAll'
while ( $row = $resultBySQL->fetch(PDO::FETCH_ASSOC); ) 
{
    $user = new User();
    $user->setUsername($row['username']);

    //...
}

PHP Snippet 3:

$rows = $resultBySQL->fetchAll(PDO::FETCH_ASSOC);

foreach( $rows as $row )
{
    $user = new User();
    $user->setUsername($row['username']);

    //...
}