AFAIK, Its not possible.

Actually its not possible , but you could make small trick to solve it , as i did in some projects i was working on
first i made hidden inputs in my php file , then in the javascript i getElemetbyIDvalue , and then i assign the value to the hidden input
so when i submit the form , the post variables will be my javascript vars
ill make example and post it

If u want to use the java script variable in php code which is reside in the same file perhaps its not possible. U write the code in separate php file, and call that file as :
window.location.href = "filename.php?Value1=" +javaScriptVariable;
and in php u can use this variable value as:
$idValue= $_GET['Value1'];

Yes, just load the variable into http's GET, POST, or REQUEST arrays. It's how AJAX applications communicate with the server.