Only change the quantity when product already exists in database - php

Question

I have a problem with some kind of 'webshop' I'm building
I add some products in the database but I want to only change the quantity when the product already exists.
I only don't know how to achieve this. Can somebody maybe help me with a function to check if the product already exists so I can add a +1 to ['aantal'] everytime I add a product that already exists?
the post is:
if(isset($_POST['submit-bonbon'])){
$postGewicht = $_POST['gewicht'];
$bonbonName = $_POST['product'];
$image = $_POST['afbeelding'];
$quantity = $_POST['aantal'];
$jsonData = unserialize($_SESSION['json_data']);
if(count($jsonData,1) >= 1){
$jsonData['bonbons'][] = array('gewicht' => $postGewicht,'name' => $bonbonName,'afbeelding' => $image, 'aantal' => $quantity);
} else {
$bonbonsArray[] = array('gewicht' => $postGewicht,'name' => $bonbonName,'afbeelding' => $image, 'aantal' => $quantity);
$order = array('userId' => $bonbonUser, 'bonbons' => $bonbonsArray);
$jsonData = $order;
}
$_SESSION['json_data'] = serialize($jsonData);
$orderData = $_SESSION['json_data'];
$query = mysql_query("SELECT * FROM CMS_bonbonOrder WHERE userId='$bonbonUser'");
mysql_query("UPDATE CMS_bonbonOrder SET jsonData='$orderData' WHERE userId='$bonbonUser';");
}
So I put a Json string in the database that looks like this for example:
Array
(
[bonbons] => Array
(
[0] => Array
(
[gewicht] => 16
[name] => test
[afbeelding] => amarena%2020g1.jpg
[aantal] => 1
)
[1] => Array
(
[gewicht] => 16
[name] => test
[afbeelding] => amarena%2020g1.jpg
[aantal] => 1
)
[2] => Array
(
[gewicht] => 16
[name] => test
[afbeelding] => amarena%2020g1.jpg
[aantal] => 1
)
[3] => Array
(
[gewicht] => 18
[name] => Testbonbon 2
[afbeelding] => bresil%2017g1.jpg
[aantal] => 1
)
[4] => Array
(
[gewicht] => 16
[name] => test
[afbeelding] => amarena%2020g1.jpg
[aantal] => 1
)
)
)
I really hope I made myself clear enough!
Thanks