How to use currency data in a php and mysql web Application?

I'm a newbie in web development Currently in building a web application for a company using php and mysql The problem is: The capital is in USD dollar and they sale thier products in AFG, the application should be updated all the time since the currency data may change frequently The only thing I know is that I should use ÀPI but I don't know what else i need, how to get and use it and from where? Any help will be appreciated. Thanks in advance.