If statement help

Im trying to build a referral system using Stripe. Stripe does not allow customers to have multiple coupons, so Im trying to use if statements to change the $coupon. The problem with my code is that it loops through each if statement and takes you to the last one i.e. $coupon = "Referral" to $coupon = "Referral10". Basically it causes the user from going from 10% for one referral to 100% for one referral. Good for the customer, bad for the business. Heres my code. Its PHP. Can anyone help? Ive tried if/else, else if, and combinations of the two, and cant get it to work properly. I want it to stop after each if statement

$coupon = $referraldata["coupon"];
    if (empty($coupon)){
        $coupon = "Referral";
    } 

if ($coupon = "Referral"){
  $coupon = "Referral2";
} 

 if ($coupon = "Referral2"){
  $coupon = "Referral3";
} 

if ($coupon = "Referral3"){
  $coupon = "Referral4";
} 

if ($coupon = "Referral4"){
  $coupon = "Referral5";
} 

if ($coupon = "Referral5"){
  $coupon = "Referral6";
} 

if ($coupon = "Referral6"){
  $coupon = "Referral7";
} 

if ($coupon = "Referral7"){
  $coupon = "Referral8";
} 

if ($coupon = "Referral8"){
  $coupon = "Referral9";
} 

if ($coupon = "Referral9"){
  $coupon = "Referral10";
}