How to Increment the quantity using ajax, php sessions?

session_start();

    $id=$_POST['id'];

    $productByCode= Yii::app()->db->createCommand("SELECT * FROM products where id='".$id."'")->queryRow();
    $itemArray = array($productByCode["id"]=>array('ItemName'=>$productByCode["ItemName"], 'id'=>$productByCode["id"], 'price'=>$productByCode["price"], 'quantity'=>$quantity));


if(!empty($_SESSION["cart_item"])) {
    if(in_array($productByCode[0]["code"],$_SESSION["cart_item"])) {
        foreach($_SESSION["cart_item"] as $k => $v) {
        if($productByCode[0]["code"] == $k)
            $_SESSION["cart_item"][$k]["quantity"] = $_POST["quantity"];
        }
    } 
    else {
        $_SESSION["cart_item"] = array_merge($_SESSION["cart_item"],$itemArray);
    }
} 
else {
    $_SESSION["cart_item"] = $itemArray;
}


if(isset($_SESSION["cart_item"])){
    $item_total = 0;
?>  
<table cellpadding="10" cellspacing="1">
<tbody>
<tr>
<th><strong>Name</strong></th>
<th><strong>Quantity</strong></th>
<th><strong>Price</strong></th>
</tr>   
<?php       
    foreach ($_SESSION["cart_item"] as $item){
        ?>
                <tr>
                <td><strong><?php echo $item["ItemName"]; ?></strong></td>
                <td><?php echo $item["quantity"]; ?></td>
                <td align=right><?php echo "$".$item["price"]; ?></td>
                </tr>
                <?php
        $item_total += ($item["price"]*$item["quantity"]);
        }
        ?>

<tr>
<td colspan="5" align=right><strong>Total:</strong> <?php echo "$".$item_total; ?></td>
</tr>
</tbody>
</table>        
  <?php
}

i have above example code, it shows result when we click on add to cart product, n stores in session

Name    Quantity    Price
Banana  1           $50.00
Banana  1           $50.00
Banana  1           $50.00
Banana  1           $50.00
Banana  1           $50.00
Banana  1           $50.00
                 Total: $300

But i want to display like this, when we click on add to cart button on product, only quantity increase not repeat the products.

Name    Quantity    Price
Banana  6           $50.00

                     Total: $300