How can I get the current page's full URL on a Windows/IIS server?

Maybe, because you are under IIS,

$_SERVER['PATH_INFO']

is what you want, based on the URLs you used to explain.

For Apache, you'd use $_SERVER['REQUEST_URI'].

$pageURL = (@$_SERVER["HTTPS"] == "on") ? "https://" : "http://";
if ($_SERVER["SERVER_PORT"] != "80")
{
    $pageURL .= $_SERVER["SERVER_NAME"].":".$_SERVER["SERVER_PORT"].$_SERVER["REQUEST_URI"];
} 
else 
{
    $pageURL .= $_SERVER["SERVER_NAME"].$_SERVER["REQUEST_URI"];
}
return $pageURL;

For Apache:

'http'.(empty($_SERVER['HTTPS'])?'':'s').'://'.$_SERVER['SERVER_NAME'].$_SERVER['REQUEST_URI']

You can also use HTTP_HOST instead of SERVER_NAME as Herman commented. See this related question for a full discussion. In short, you are probably OK with using either. Here is the 'host' version:

'http'.(empty($_SERVER['HTTPS'])?'':'s').'://'.$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI']

For the Paranoid / Why it Matters

Typically, I set ServerName in the VirtualHost because I want that to be the canonical form of the website. The $_SERVER['HTTP_HOST'] is set based on the request headers. If the server responds to any/all domain names at that IP address, a user could spoof the header, or worse, someone could point a DNS record to your IP address, and then your server / website would be serving out a website with dynamic links built on an incorrect URL. If you use the latter method you should also configure your vhost or set up an .htaccess rule to enforce the domain you want to serve out, something like:

RewriteEngine On
RewriteCond %{HTTP_HOST} !(^stackoverflow.com*)$
RewriteRule (.*) https://stackoverflow.com/$1 [R=301,L]
#sometimes u may need to omit this slash ^ depending on your server

Hope that helps. The real point of this answer was just to provide the first line of code for those people who ended up here when searching for a way to get the complete URL with apache :)

$_SERVER['REQUEST_URI'] doesn't work on IIS, but I did find this: http://neosmart.net/blog/2006/100-apache-compliant-request_uri-for-iis-and-windows/ which sounds promising.

Use this class to get the URL works.

class VirtualDirectory
{
    var $protocol;
    var $site;
    var $thisfile;
    var $real_directories;
    var $num_of_real_directories;
    var $virtual_directories = array();
    var $num_of_virtual_directories = array();
    var $baseURL;
    var $thisURL;

    function VirtualDirectory()
    {
        $this->protocol = $_SERVER['HTTPS'] == 'on' ? 'https' : 'http';
        $this->site = $this->protocol . '://' . $_SERVER['HTTP_HOST'];
        $this->thisfile = basename($_SERVER['SCRIPT_FILENAME']);
        $this->real_directories = $this->cleanUp(explode("/", str_replace($this->thisfile, "", $_SERVER['PHP_SELF'])));
        $this->num_of_real_directories = count($this->real_directories);
        $this->virtual_directories = array_diff($this->cleanUp(explode("/", str_replace($this->thisfile, "", $_SERVER['REQUEST_URI']))),$this->real_directories);
        $this->num_of_virtual_directories = count($this->virtual_directories);
        $this->baseURL = $this->site . "/" . implode("/", $this->real_directories) . "/";
        $this->thisURL = $this->baseURL . implode("/", $this->virtual_directories) . "/";
    }

    function cleanUp($array)
    {
        $cleaned_array = array();
        foreach($array as $key => $value)
        {
            $qpos = strpos($value, "?");
            if($qpos !== false)
            {
                break;
            }
            if($key != "" && $value != "")
            {
                $cleaned_array[] = $value;
            }
        }
        return $cleaned_array;
    }
}

$virdir = new VirtualDirectory();
echo $virdir->thisURL;