Source

File: wp-admin/includes/misc.php

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;
}