buy coins as user

Hi everyone, i made a website with register and login, and i would like the user could buy coins from my website, 
for example: you pay me 1$ through paypal and immediately you recieve in your user account 10 coins.
is tha possible? how could i do it? any helper or library that i could use? 
and i should create a table for the coins? or use directly the user table from my DB 
Thanks.

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