Starting Online Service dont know were to begin!!?

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I am pretty new to PHP and Sql just learning from examples and trying it out. I am also new to the forums. I am starting an online rental service and i have many questions that if possible could be answered.

 

PLEASE READ ALL, before posting reply

 

The site will work like this.. You create an account. all ur info gets added to the database. The database will be hooked up with quickbooks or peachtree or sumtin.. for accounting. You select the item you would like to rent and choose the min about of time you would like it. Say 5 dollars a week. Then if we have it in our invantory it will move you on to the next page, to pay through paypal and your shipping method.

 

I need ideas on how to make it so i can keep track of the always coming in and out items.

if ther is only one item left and both people visit the site at the same time i cant have them both paying for it and only ship to one. Each item in the database will be taken its serial number, maybe a pic of its condition,and notes, and item number. The computer will need to be able to select an item from inventory and tell us who to send it to. Will be using quickbooks, or some other similar program linked to out database keep track of payments.

i need a way to link an item that is AWAY to the members account that is renting it. So they can go online and view details on the item.. *The computer will also need to calculate how much they pay (by the week) if there min is 1 month but they find out they need the item longer they can go online and add a few weeks, and the computer will calculate the payments.

I dont know how to do the last part since every month has different amount of days.

 

This will be an automated system and dont know if PHP and SQL is good for it. i think it is.. but i need help on how this would work. can some one explain how the SQL would be set up or how it would be. I really need help/support on this project. Thanks in advance.