Source code:

<?php	
	if( $_POST )
	{
		if ($_POST['XSRF_filter'] != $_COOKIE['XSRF_filter_cookie'])
		{
			echo "You're trying to do an XSRF attack! Can't allow you to post this form";
			echo "<p>form value: " . $_POST['XSRF_filter'];
			echo "<br>cookie value: " . $_COOKIE['XSRF_filter_cookie'] . "</p>";  
		}
		else
		{
			//process form
			echo "your form post has been processed :)";
		}
	}
	else
	{
		$randomString = md5(rand(0,100000));

		setcookie("XSRF_filter_cookie",
				  $randomString,
				  time() + 3600,
				  "/",
				  ".ideadrought.com",
				  0);
		
		?>
			<form action="" method="POST">
				Change this to create an error :)
				<br><input type="text" name="XSRF_filter" value="<? echo $randomString; ?>">
				<br>...other form elements
				<br><input type="submit" name="submit" value="submit">
			</form>
		<?
	}
	?>