php cannot be executed from within a page. That would be a huge security risk. Read the ■■■■ manual - so to say just replicate things and then you will see. You need to enter php via a shortcode.
And the temporary solution is to disable encryption. Simple as that/hard as that. The encryption is not about any personal data - but people could steal your paypal account data for credit card testing stuff - but then this as I said already doesn’t make sense on button payments. And you cannot use it to forge pro-forms.
The main problem is that anyone tech savy without encryption can check out for 0 whatever currencty he likes with 0.01 whatever currency subscription price and whatever he likes up to 5 years subscription period and initial period also whatever he likes.