Setting Custom HTTP Headers in Flash

I needed to set a custom HTTP header in Flash. ActionScript 3 makes this easy, or so I thought. Here was my original prototype code:

import flash.net.navigateToURL;
private function href(url:String):void
{
	var req:URLRequest = new URLRequest(url);
	var header:URLRequestHeader = new URLRequestHeader("Koostoom", "tmttos");
	req.requestHeaders.push(header);
	navigateToURL(req);
}

Like I said, this is easy. To verify it was working, I wrote a quick prototype using PHP:

$headers = getallheaders();
foreach ($headers as $name => $value) {
   echo "[$name] = $value<br/>\n";
}

I think I copied this out of the PHP manual pretty much. So I tested the above code and everything worked, right? I wouldn't be writing this post if that was the case!

I tried the above in Firefox and Safari. My usual test plan is to make sure it works in those two browsers first, and then deal with IE. In this case it didn't work in either browser. I knew that some HTTP headers were off-limits in Flash, so just for just kicks I tried changing Koostom to Referrer. This should have caused an exception, but of course it didn't. It did nothing.

Finally, I found the answer through experimentation. You have to do an HTTP POST with form data in order to get the custom header sent:

import flash.net.navigateToURL;
private function href(url:String):void
{
	var req:URLRequest = new URLRequest(url);
	// begin stupid hack
     req.method = "POST";
	var formVars:URLVariables = new URLVariables();
	formVars.blah = "blue";
	req.data = formVars;
     // end stupid hack
	var header:URLRequestHeader = new URLRequestHeader("Koostoom", "tmttos");
	req.requestHeaders.push(header);
	navigateToURL(req);
}

Now the PHP script showed my Koostoom header...

I don't know if this is a bug with the Flash player, or the browsers. I don't think it is part of the HTTP spec, i.e. I think custom headers are just as valid in an HTTP GET as in an HTTP POST.