how to assign javascript variable value to php variable

You should see these links:

Assign Javascript value to PHP variable

Passing Javascript vars to PHP

Or you can use AJAX request or POST and GET methods to achieve this.

Snippet below may helpful for you:

<?php 
   if(isset($_POST['isSubmit']))
   {
      // do something here
      echo $_POST["name"];
   }
?> 

<form action="<?php $_SERVER['PHP_SELF'];?>" method="POST"> 
Your name: <input type="text" name="name" /> 
<input type="Submit" value="Submit" name="isSubmit"> 
</form> 

Try using ajax with jQuery.post() if you want a more dynamic assignment of variables.

The reason you can't assign a variable directly is because they are processed in different places.

It's like trying to add eggs to an already baked cake, instead you should send the egg to the bakery to get a new cake with the new eggs. That's what jQuery's post is made for.

Alert the results from requesting test.php with an additional payload of data (HTML or XML, depending on what was returned).

$.post( "test.php", { name: "John", time: "2pm" })
  .done(function( data ) {
    alert( "Data Loaded: " + data );
  });

Javascript will be interpreted in Client's browser and you can not assign it to PHP variable which is interpreted on SERVER .

Feasible Solution : You can submit the Javascript value via ajax or through form submit.

I guess you can use cookies for it.

1) First add a cookie jquery plugin.

2) Then store that window width in a cookie variable.

3) Access your cookie in PHP like $_COOKIE['variable name'].

http://www.w3schools.com/php/php_cookies.asp

You can not do this directly for that you should use ajax. For reference you can refer this link