How hard would it to be to write a fully functional P.O.S. software (point of sale) in php to be run off of a web server so many users can be logged on at once and get the most up-to-date invnetory readings and sales? i looked on the net. and i can not find one only invoice or inventory control systems.

so how hard would it be for me to make a entire site for POS software and for multiple users.

I planned on incorperating MySQL databases. for it.

How would i make a form feild for it to do the following...

if some one enters data into a field (stock #) upon complettion and clicking or tabbing out of it how would i write it to automaticly parse the information for a mysql db and input it in the fields next to it....

ie sales enter stock # 123 and the data from mysql row 123 is shown in the corrisponding table cells item name, discription, price, service. then when the users inputs the quantity it updates the final price. i can do this in excell and access but trying to learn in php or js.