Ajax in JavaScript to pass to PHP

I’m using radio buttons (‘countries’) to open dropdown-box of its corresponding ‘cities’.
The variable ‘radioPressed’ detects the selected radio button and I need to pass its value inside my JavaScript to a PHP, as follows:

Code:

<script> var radioPressed = 123; <?php $what_is_myVar = radioPressed ; ?> </script>

Since the solution can be complicated, I would ask to receive an advice only regarding the main steps of how to pass the value of 'radioPressed' into $what_is_myVar

Thanks a lot for any assistance,

Here's an example. I've used "x" instead of radioPressed for the variable.

Code:

<!doctype html> <html> <head> <meta http-equiv="content-type" content="text/html; charset=utf-8" /> <title>Title</title> <meta name="language" content="en" /> <script type="text/javascript"> //<![CDATA[ function passVariable(){ // set your variable var x = '123'; // get the current url and append your variable var url = document.location.href + '?x=' + x; // to prevent looping, check to make sure your current url contains a common string var exists = document.location.href.indexOf('?x='); if(exists < 0){ // redirect passing your variable in window.location = url; } } //]]> </script> </head> <body onload="passVariable()"> <?php echo $_REQUEST['x']; ?> </body> </html>

Thanks very much for your professional reply, works perfect but I need to have the PHP variable inside JavaScript, because the PHP code for opening the dropdown-box of matching cities is inside the JavaScript (see below my working script):

Code:

<script type="text/javascript"> $(document).ready(function(){ $('#locationTree input:radio').change(function() { var radioPressed = $('input:radio[name=Region]:checked').val(); if(radioPressed == 123) { var listCities = $("select[name='City']"); <?php View::newInstance()->_exportVariableToView(123); ?> <?php while(has_list_cities()) { ?> $("<option><?php echo list_city_name(); ?></option>").appendTo(listCities); <?php } ?> } }); }); </script>

In the code above I need to use $_REQUEST['x'] as follows:
<?php View::newInstance()->_exportVariableToView($_REQUEST['x']); ?>

Is there any way to get the PHP variable inside JavaScript?

Your help is greatly appreciated,