I was contracted to fix a broken website, and in order to do so, I had to implement IPN with PayPal. I found an example site on Formget. The tutorial was useful for learning how to start one, but I found there were many bugs in the code they offered for download. I got something started over on http://paypal-ipn-php.steemfiles.com where you can find a sandbox test site for buying headphones.
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From @leprechaun