title: PHP HTTP Referrer
body: <p>I have a page which accepts POSTs from a remote site. I would like to detect the domain that these POSTs are coming from. I realize that it can be spoofed but it is better than nothing.
  I have tried accessing the HTTP_REFERER variable but it just returns null.</p>
  
  <p>The page accepts POSTs from sources like PayPal (instant payment notifications) and other payment gateways.</p>
  
  <p>How can I get the referring call?</p>