Calculate quantity based on different text values - php

Question

Good day
I'd be grateful for some expertise with this issue of mine.
Assume we have the following variables in a hypothetical situation:
$uom = (either "PLT" or "CRT")
$prow = item ID in cart (starts from 1 and goes up depending on how many items)
$quantity = quantity of each item
$Name = item name
$quantityplt = new variable for PLT quantity
$quantitycrt = new variable for CRT quantity
This information is acquired for each product from a cart shopping system on the website. So once the user adds an item to the cart, that information is displayed for each item in the cart that they chose.
I have two text inputs which need to display the total_quantity for all the items: depending on whether or not the $uom is "CRT" or "PLT".
<input type="text" id="plt_sum" readonly/>
<input type="text" id="crt_sum" readonly/>
Now, I need to calculate the $quantity depending on $uom.
It must then display the value in either "#plt_sum" or "#crt_sum"...
I'm not confident in using loops so I don't really know how to:
<?php
foreach($prow){
if($uom=="PLT"){
$quantityplt value == (perform $quantity addition WHERE $uom is "PLT");
echo $quantityplt (in the #plt_sum text input);
}
elseif($uom=="CRT"){
$quantitycrt value == (perform $quantity addition WHERE $uom is "CRT");
echo $quantitycrt (in the #crt_sum text input);
}
}
?>
I do hope this piece of php makes any sense at all. I realise my syntax and everything else is most likely wrong. I'm still very new to it and learning things every day.
Thank you for reading and I appreciate any help.