checking item availability by date

[civciv03]

I am building a page to check availability of items for hire before allowing the customer to add the item in their shopping cart.

HIRE_ITEM table holds item_id, hire_no, start_date, end_date information for each hire item in hire agreement.

I have a PHP form which asks users to enter their intended hire period to check which items will be available between those days. So the form data is :

$HTTP_POST-VARS [‘start_date’]

$HTTP_POST-VARS [‘end_date’]

I wrote the following sql query to check if the given end_date prior to hire_item.start_date OR if the given start date after hire_item.end_date

SELECT DISTINCT item_id
FROM hire_item
WHERE hire_item.start_date > $HTTP_POST-VARS [‘end_date’]
OR hire_item.end_date < $HTTP_POST-VARS [‘start_date’]

This doesn't work for various reasons, can anyone help please. I am new to MySQL and need to get this done soon.
Many Thanks
Di.