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'.