<?php
switch ($_GET['lang']) {
	case 'ru':
		$lang = 'en';
		break;
	case 'en':
		$lang = 'ru';
		break;
	default:
		$lang = 'en';
		break;}
?>

<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
</head>
<body>
	<a href="/?lang=<?=$lang?>"><?=$lang?></a>
</body>
</html>