PayPal Automation

What I need is very simple. When someone comes to my web site and buys or donates money for an Item on my web page, I need my site to transact with PayPal’s website and get the transaction information, write it to MySQL database and then send me and the buyer an email with the information. The Email is sent through Google’s Email server. I would like the code briefly commented throughout stating what is done and not scrambled up. Thank You.