card like visa card or shopping card

I have a website for shopping I want to make own shop card I give this card to someone who buy it from me when he register in the site and put the number in from the card he will have an amount in my website like when he register first time his account have 0$ when he buy a card from me as example amount is 50$ his account charge with 50$ and he can buy in my website and when he buy from my site his amount will be min from his account I mean when he buy a product with 20$ his account will be 30$ Instead of 50$ as example I want all this in php and mysql