I'm testing PayPal as a payment method for my website in sandbox mode. My requirement is, an email must be send to the merchant after each transaction. I am using the PHP SDK. How is this possible?
This is the configuration that I have seen for notifications in paypal.
PayPal already sends its own emails to the merchants after each transaction completes. In sandbox, you can see the notification methods in the "Notifications" tab of https://developer.paypal.com
If you want to generate your own emails in addition to the PayPal emails, you should use a server-side integration. Here is the front-end UI: https://developer.paypal.com/demo/checkout/#/pattern/server
You'll need two corresponding routes on your server, one for 'Set Up Transaction' and one for 'Capture Transaction', documented here: https://developer.paypal.com/docs/checkout/reference/server-integration/
That last route of yours can send your email on success.