Solve math equation in php

I'm trying to solve for an APR and the formula is not working. It is in PHP, I have a formula that successfully solves for the payment with the interest rate from an array. If I have a payment I want to be able to solve for the interest rate variable. Here is the formula below: $payment = number_format($current_loan_amount * ($csv[ $x ][0]/100/12) / (1-(pow((1 + $csv[ $x ][0]/100/12),- 360))), 2); For testing purposes, payment is $886.55 current_loan_amount is $200,000 I need a variable set for $csv[ $x ][0] and solved with code. Both payment and current_loan_amount will change so I don't need an answer, I just need it setup in php.