PHP sum each loop value - php

Question

I wanted to make a simple calculations to summarized what I purchased.
Using $_GET every time the value is updated it should save in an array then when 'start' is executed, it gives the sum. Sorry the codes here are just googled and I'm not really a programmer. I don't know how to combine the two sets of code (array + sum).
$number = $_GET ['input'];
$arr = array ($number);
$data = array($arr);
foreach ($tareas as $tarea) {
$data[] = $tarea;
}
var_dump($data);
$sum = 0;
foreach($group as $key=>$arr) {
$sum+= $arr;
}
echo $sum;

Answer 1

So I got this code from withinweb.com and modified to make it simpler.
<?php session_start();
$products = array($_GET["prod"]);
$amounts = array($_GET ["cost"]);
if ( !isset($_SESSION["total"]) ) {
$_SESSION["total"] = 0;
for ($i=0; $i< count($products); $i++) {
// $_SESSION["qty"][$i] = 0;
$_SESSION["amounts"][$i] = 0;
}
}
//---------------------------
//Reset
if ( isset($_GET['reset']) )
{
if ($_GET["reset"] == 'true')
{
unset($_SESSION["qty"]); //The quantity for each product
unset($_SESSION["amounts"]); //The amount from each product
unset($_SESSION["total"]); //The total cost
unset($_SESSION["cart"]); //Which item has been chosen
}
}
//---------------------------
//Add
if ( isset($_GET["add"]) )
{
$i = $_GET["add"];
$qty = $_SESSION["qty"][$i] + 1;
$_SESSION["amounts"][$i] = $amounts[$i] * $qty;
$_SESSION["cart"][$i] = $i;
$_SESSION["qty"][$i] = $qty;
}
//---------------------------
//Delete
if ( isset($_GET["delete"]) )
{
$i = $_GET["delete"];
$qty = $_SESSION["qty"][$i];
$qty--;
$_SESSION["qty"][$i] = $qty;
//remove item if quantity is zero
if ($qty == 0) {
$_SESSION["amounts"][$i] = 0;
unset($_SESSION["cart"][$i]);
}
else
{
$_SESSION["amounts"][$i] = $amounts[$i] * $qty;
}
}
//cart
if ( isset($_SESSION["cart"]) ) {
$total = 0;
foreach ( $_SESSION["cart"] as $i ) {
echo '' . $products[$_SESSION["cart"][$i]] . ' - ' . $_SESSION["amounts"][$i] . '<br>';
$total = $total + $_SESSION["amounts"][$i];
}
$_SESSION["total"] = $total;
echo'
<br>
Total : ' . $total . '
';
}
?>
When I input ?add=0&prod=apple&cost=100, it gives me:
apple - 100
Total : 100
But when I add another session, ?add=1&prod=orange&cost=200 it doesn't give the right answer.
orange - 100
- 0
Total : 100
It should return me this value, I'm puzzled where could be the error.
apple - 100
orange - 200
Total : 300
Yes, I'm not a coder, but trying to solve a big problem.. :) Thanks for those who help.