It seems that you are using GET method, so (as MANUAL said ;-) )
"HTTP_GET_VARS
An associative array of variables passed to the current script via the HTTP GET method."
So you can read borrar variable like that:
$HTTP_GET_VARS["borrar"]
or using simply $borrar (it depends on your PHP configuration, if you have the register_globals and track_vars options turned on)
Best regards
bunff