Change inventory to work in FIFO in PHP Order Manager

We currently have a simple php and mysql software to manage our sales orders. It manages inventory in a very simple way, just add and substract from total quantity When we need to add inventory, we just add the amount to the inventory displayed on screen, and we manually calculate the average cost. What we need is the inventory and cost to work in FIFO, and give option to work in average cost. We need someone to modify the way inventory works so we can view past inventory aditions, substractions, etc. Right now we have a screen where a list of products is displayed, along with its cost, quantity, and price. All of which can be edited. We need that to change to a screen where a list of products is displayed with total quantity available, current cost, and price. When clicked on the product name or row, a new screen is displayed with the inventory batch history. There I should be able to add inventory quantity and cost, view available inventory batch, and past depleted inventory. For example: Product List PID Product Quantity Cost Price 1 USB Memory 1GB 126 $10 $12 [Edit] [Del] 2 Computer Monitor14 $150 $170 [Edit] [Del] 3 Mouse 42 $3 $5 [Edit] [Del] Example of inventory screen: Product: USB Memory 1GB [Add] Date Quantity Available Cost 20/05/2008 100 100 9.8 [Edit] 15/04/2008 50 26 10.0 [Edit] 02/04/2008 90 0 11.0 [Edit] 25/03/2008 -2 0 12.0 [Edit] In this example, the USB Memory 1GB, there are 126 units available, the current FIFO cost is $10. When we look into the inventory history, the units from 02/04/2008 are depleted, now the system is using inventory from 15/04/2008 until they get depleted, the system then will use the 20/05/2008 inventory with $9.8 cost. Notice that in product price is not set in inventory screen, price should be able to be set somehow in the product list. Once a coder is selected, the source code will be given. We accept suggestions or ideas. Thanks.