PHP - array_push is replacing instead of add to SESSION Variable

<?php 
session_start();
$pid = $_GET['pid']; 
$ptype = $_GET['ptype'];
$_SESSION = array();
$_SESSION['cart_items'] = array();

if (isset($_GET['add_cart']) && !empty($_GET['add_cart'])) {
  // Add new data to Session var

  $newdata = array($pid , $ptype, 1 );

  array_push($_SESSION['cart_items'], $newdata);
}

echo '<pre>';
var_dump($_SESSION);
echo '</pre>';

?>

array_push replaces the data already in the $_SESSION with $newdata variable in the $_SESSION instead of adding it.

For example:

I enter the url: ?pid=1&ptype=CH-&add_cart=Add+to+Cart And the array looks like this:

array(1) {
  ["cart_items"]=>
  array(1) {
    [0]=>
    array(3) {
      [0]=>
      string(1) "1"
      [1]=>
      string(3) "CH-"
      [2]=>
      int(1)
    }
  }
}

That's great. But when I enter another url like: ?pid=1&ptype=CPU-&add_cart=Add+to+Cart The array looks like this:

array(1) {
  ["cart_items"]=>
  array(1) {
    [0]=>
    array(3) {
      [0]=>
      string(1) "1"
      [1]=>
      string(4) "CPU-"
      [2]=>
      int(1)
    }
  }
}

instead of this:

array(1) {
  ["cart_items"]=>
  array(1) {
    [0]=>
    array(3) {
      [0]=>
      string(1) "1"
      [1]=>
      string(3) "CH-"
      [2]=>
      int(1)
    }
    [1]=>
    array(3) {
      [0]=>
      string(1) "1"
      [1]=>
      string(4) "CPU-"
      [2]=>
      int(1)
    }
  }
}

It replaces the data that was already in the Session. I want it to add to it. How do I do so?

Thanks in advance!