CODE EXAMPLE FOR PHP

how to sum values of two product which are same gst rate and this product are come from foreach loop in php

$itemsArray = array();
        $i = 1;
        foreach($request['cost'] as $cost){
            $itemsArray[$i] = $cost;
            $i++;
        }

$data = new model;
$data->cost =  json_encode($itemsArray);