undefined function http_get_request_headers()

I'm used the example in the readme file to try to begin writting a REST call, but I'm getting the following error when I browse to the call:

Fatal error: Call to undefined function http_get_request_headers() in /home/sites/drupal6.vmtllc.com/htdocs/sites/all/modules/rest_provider/rest_provider.module on line 161

Please let me know what server configurations I need to make to enable this http_get_request_headers function.

Thank you.