Do I use java script or php for adding or subtracting a given amount from a database

Question

so I'm beginning to understand the use of php and js and I have a situation. I created an inventory system but I'm still unclear on creating sales. my idea was to have a sale form to send the updated information back to the dB (eg new stock amount and the saleid). however, I would like to implement a counter so when the item that was added can be (eg 1 pen and when the add button next to the item was pressed it would be 2 pens). after the sale it would then be deducted from the inventory (10 pens - 2)

Answer 1

I think JavaScript and PHP can both be of use for that, I remember a friend here created a tutorial on how to use JavaScript to send request to a PHP back-end code, and receive response in JSON (if I'm not mistaken), shamefully that friend is no longer here, so the tutorial in the code can no longer be accessed

Answer 2

would this increment item/decrement item be php or java script? thanks to anyone who can clear this up for me. <3