How to Pass and Post Back User Defined Variables in Paypal Ipn

With paypal api, payment operation such as buy, donate, subscribe has never been easier. For simple cases, you just need to define a button inside paypal and embed it in your website, usually on blog sites that ask for donation. For cases that requires callback event, so that the website can show a message on whether the transaction is successful or not:

<form method="post" action="https://www.sandbox.paypal.com/cgi-bin/webscr">
<input type="hidden" value="_donations" name="cmd">  
<input type="hidden" value="youremail" name="business"> 
<input type="image" border="0" alt="" name="submit" src="https://www.paypal.com/en_US/i/btn/btn_xpressCheckout.gif">
<input type="hidden" value="My Item" name="item_name">
<input type="hidden" value="10" name="amount">
<input type="hidden" value="USD" name="currency_code">
<input type="hidden" value="http://yourreturnurl.com" name="return">
<input type="hidden" value="http://yourcancelurl.com" name="cancel_return">
<input type="hidden" value="http://yournotificationurl.com" name="notify_url">
</form>

You can refer to this website for paypal variables:
https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_html_Appx_websitestandard_htmlvariables

IPN variables: https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_html_IPNandPDTVariables

But in your notify url, what if you want to post user defined variables such as user id, etc? In this case I use the optional parameters from paypal on0, on1, os0, os1. You can see them in the first website I've mentioned above. So our form becomes

<form method="post" action="https://www.sandbox.paypal.com/cgi-bin/webscr">
<input type="hidden" value="_donations" name="cmd">  
<input type="hidden" value="youremail" name="business"> 
<input type="image" border="0" alt="" name="submit" src="https://www.paypal.com/en_US/i/btn/btn_xpressCheckout.gif">
<input type="hidden" value="My Item" name="item_name">
<input type="hidden" value="10" name="amount">
<input type="hidden" value="USD" name="currency_code">
<input type="hidden" value="http://yourreturnurl.com" name="return">
<input type="hidden" value="http://yourcancelurl.com" name="cancel_return">
<input type="hidden" value="http://yournotificationurl.com" name="notify_url">
<input type="hidden" value="user_id" name="on0">
<input type="hidden" value="1" name="os0">
</form>

And in your notify url, you should have a post variables: option_name1=user_id option_selection1=1