how to calculate credit card interest / how long it will take to pay it off?

I need an EQUATION or two. I’m trying to program a credit card debt calculator in PHP to basically tell me this as a result…..

if you currently owe $80,000 in credit cards, and your average interest rate is 20%,

You will pay $159,196.96 in interest,
your total payment will be $239,196.96

and it will take you 54.33 years by paying the minimums.

the only part that i know for sure is to add the two numbers together to get the total amount.. beyond that i’m kind of lost, mostly because i don’t understand how credit card interest works on a deep level.

I understand the “simple interest” model, which is NOT what credit cards work on.. most of the stuff i find on the net is for mortgages and simple interest. that doesn’t help me in this case.

thanks in advance!!

the equations themselves will help.