SQL Error has me baffled

[Guest]

I tried to search for some similar topic, but as usual, the search was... less than fruitful. Anyway, the following error is driving me nuts. I can't seem to find out how to fix it

Call to undefined function: get_quantity()

it says it's undefined, it's coming from the app top, but the thing is defined in the shopping_cart class (to the best of my knowledge). Anyone have an insight? It pops up right after I click the "add product to cart" button from product_info. I get a similar error when I try to go directly to shopping cart. Help