paypal money request weird api

I found a weird script. It uses weird paypal api for mone开发者_Go百科y requesting. It uses redirect to page with all parameters included by GET request

location:https://www.paypal.com/xclick?business=$aset[PayPalEmail]&item_name=$a2[ads] ads for $a2[days] days&first_name=$a1[FirstName]&last_name=$a1[LastName]&email=$a1[email]&item_number=1&custom=$MyID|$_GET[SelectedPackage]|$flag&amount=$a2[price]&notify_url=http://$_SERVER[HTTP_HOST]notify.php&return=http://$_SERVER[HTTP_HOST]"

I didn't find any api documentation about this way of money requesting. Who can explain how it works and how i can debug it with paypal sandbox ?

You might wanna have a look at.

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