Unique Alpha-Numeric Code Generation

I am selling a physical product and will be including unique alpha-numeric codes in each box that will allow the user to go online, enter their code, and receive virtual currency in exchange for their code. All I need right now is an online PHP script that allows me to generate thousands of codes and specify how much virtual currency each one is worth at generation time as well as a script for checking a code. For example, a person will purchase a product, get the code, enter the code online, and receive $25 of virtual currency in exchange. Someone else will purchase a larger product, get the code, enter the code online, and receive $50 of virtual currency in exchange. I need each code to be unique (so they can only be used once) and I need to be able to generate thousands of codes myself using this utility. I assume the codes themselves or some generating key(s) will be stored in a database. I am not an expert, but I think it would be bad design to have the script hit the database for every code generated to check to see if the code already existed. A different method must be used. Once the codes are generated, I need the script to save the codes in a database as well as output the codes to a file. I think the best way to output to a file is a text file with one code per line. Probably naming the file with the date & time and the amount of the virtual currency the batch of codes is worth. (i.e. [login to view URL] = 10/20/2008 14:27 $25) Finally, I need this project completed at the very latest by October 24, but preferably sooner. I need to start printing the codes ASAP.