PDO has nothing to do with Slim. Slim does not pretend how you should use PDO. You might as well use another database library.
The question is, why do you need a stored procedure? You can also use a normal INSERT statement.
Example:
$row = [
'username' => 'bob',
'email' => 'bob@example.com'
];
$sql = "INSERT INTO users SET username=:username, email=:email;";
$status = $pdo->prepare($sql)->execute($row);
if ($status) {
$newId = $pdo->lastInsertId();
}
PS: What do you get if you call this?
$row = $pdo->query('SELECT LAST_INSERT_ID() AS id')->fetch();
print_r($row);