Upgrade script from PHP 5.6 to 7.2 appear error for Paypal

Hello

My website is about to people can buy some software from my website. There is one icon on site where user can see “buy now”.When user click on that icon then he will be redirected to Paypal,and then make a payment.

Now the problem is when i was switch from PHP 5.6 to PHP 7.2 in Whm that action just give to me blank screen.So that mean that for 7.2 PHP version can’t finish payment with Paypal.
SEMrush

Just to mention script work great with other functions as Home and contact page,and is full in function in PHP 7.2 but only payment on icon “Buy now” can’t be finished.

How to fix that in some file?

Thanks