index.php

<html>
	<head>
		<title>Playground</title>
		<script type="text/javascript" src="https://code.jquery.com/jquery-2.1.1.min.js"></script>
		<script type="text/javascript">
			$(document).ready(function(){
				var value = 12;
				$('a').click(function(){
					$.ajax({
						url: "/handler.php",
						data: { key: value },
						type: "GET",
						beforeSend: function(xhr){xhr.setRequestHeader('Referer', 'lalala');},
						success: function(data) { alert('data: ' + data); }
					});
				});
			});
		</script>
	</head>
	<body>
		<a href="#">Click me!</a>
	</body>
</html>