The short answer is you can't.
I don't know any PHP syntax, but what I can tell you is that PHP is executed on the server and JavaScript is executed on the client (on the browser).
You're doing a $_GET, which is used to retrieve form values:
The built-in $_GET function is used to collect values in a form with method="get".
In other words, if on your page you had:
<form method="get" action="blah.php">
<input name="test"></input>
</form>
Your $_GET call would retrieve the value in that input field.
So how to retrieve a value from JavaScript?
Well, you could stick the javascript value in a hidden form field...
<script type="text/javascript" charset="utf-8">
var test = "tester";
// find the 'test' input element and set its value to the above variable
document.getElementByID("test").value = test;
</script>
... elsewhere on your page ...
<form method="get" action="blah.php">
<input id="test" name="test" visibility="hidden"></input>
<input type="submit" value="Click me!"></input>
</form>
Then, when the user clicks your submit button, he/she will be issuing a "GET" request to blah.php, sending along the value in 'test'.