Warning: Illegal string offset

Warning: Illegal string offset

I'm trying to set session in my program, after the user has logged with
his account. If user input good email and password, then program set some
variables, like this:
$_SESSION['user']['id'] = $row['id'];
$_SESSION['user']['email'] = $email;
$_SESSION['user']['admin'] = $row['admin'];
$_SESSION['logged'] = true;
The problem is, that I'm getting this warning from first three rows of
code above:
Warning: Illegal string offset 'id' in C:\Program Files
(x86)\Zend\Apache2\htdocs\OOPeshop\user\User.php on line 193
So I checked, if all the variables are set, so i add to my code this two
lines:
var_dump($row);
var_dump($email);
but it was looking alright, I was getting output
array(2) { ["id"]=> string(2) "14" ["admin"]=> string(1) "0"}
string(22) "foobar@gmail.com"
So can someone explain me where is the problem? I checked similar
question, but found no solution to my problem.
I have tried this:
$user = array('id' => $row['id'], 'email' => $email, 'admin' =>
$row['admin']);
$_SESSION['user'] = $user;
No more warnings. It set array first and then set whole array to session
variable, it worked, but i don't know where's problem in my first code, so
maybe someone with better understanding can explain it better.