iis7_add_rewrite_rule( string $filename, string $rewrite_rule )

function iis7_add_rewrite_rule($filename, $rewrite_rule) {
	if ( ! class_exists( 'DOMDocument', false ) ) {
		return false;
	}

	// If configuration file does not exist then we create one.
	if ( ! file_exists($filename) ) {
		$fp = fopen( $filename, 'w');
		fwrite($fp, '<configuration/>');
		fclose($fp);
	}

	$doc = new DOMDocument();
	$doc->preserveWhiteSpace = false;

	if ( $doc->load($filename) === false )
		return false;

	$xpath = new DOMXPath($doc);

	// First check if the rule already exists as in that case there is no need to re-add it
	$wordpress_rules = $xpath->query('/configuration/system.webServer/rewrite/rules/rule[starts-with(@name,\'wordpress\')] | /configuration/system.webServer/rewrite/rules/rule[starts-with(@name,\'WordPress\')]');
	if ( $wordpress_rules->length > 0 )
		return true;

	// Check the XPath to the rewrite rule and create XML nodes if they do not exist
	$xmlnodes = $xpath->query('/configuration/system.webServer/rewrite/rules');
	if ( $xmlnodes->length > 0 ) {
		$rules_node = $xmlnodes->item(0);
	} else {
		$rules_node = $doc->createElement('rules');

		$xmlnodes = $xpath->query('/configuration/system.webServer/rewrite');
		if ( $xmlnodes->length > 0 ) {
			$rewrite_node = $xmlnodes->item(0);
			$rewrite_node->appendChild($rules_node);
		} else {
			$rewrite_node = $doc->createElement('rewrite');
			$rewrite_node->appendChild($rules_node);

			$xmlnodes = $xpath->query('/configuration/system.webServer');
			if ( $xmlnodes->length > 0 ) {
				$system_webServer_node = $xmlnodes->item(0);
				$system_webServer_node->appendChild($rewrite_node);
			} else {
				$system_webServer_node = $doc->createElement('system.webServer');
				$system_webServer_node->appendChild($rewrite_node);

				$xmlnodes = $xpath->query('/configuration');
				if ( $xmlnodes->length > 0 ) {
					$config_node = $xmlnodes->item(0);
					$config_node->appendChild($system_webServer_node);
				} else {
					$config_node = $doc->createElement('configuration');
					$doc->appendChild($config_node);
					$config_node->appendChild($system_webServer_node);
				}
			}
		}
	}

	$rule_fragment = $doc->createDocumentFragment();
	$rule_fragment->appendXML($rewrite_rule);
	$rules_node->appendChild($rule_fragment);

	$doc->encoding = "UTF-8";
	$doc->formatOutput = true;
	saveDomDocument($doc, $filename);

	return true;
}