[resolved] If querystring exists?

[solitario]

Hello all.

<?php
switch ($_GET['lang']) {
case "es":
	$txtGreeting = array("buenas dias");
	break;
case "fr":
	$txtGreeting = array("bon jour");
	break;
default:
	$txtGreeting = array("good day");
}
?>

The above code works just fine when I click on a link such as:

<a href="index.php?lang=fr">French</a>

But if I click a link without the querystring (?lang=fr), I receive a nasty error message:

"Undefined index: lang"

Could anyone explain to me how to determine if a querystring exists BEFORE performing the switch statement?
Or, if possible, how I could switch to default if the querystring is absent?

Many thanks.