How to merge Javascript with PHP

Question

I use PHP, when necessary, I use Javascript, but the basics of the basic.

I need to capture the screen resolution of the client, I believe it is not possible in PHP because a client-side language is required (Javascript).

I’m not sure how to merge this being that my file is PHP, a function file, so I need to inject javascript in the middle of the function in PHP and pass the value of the variable in javascript to the variable in PHP, then these PHP variables enter an array.

Follow what I’m trying to:

echo "<script>
          var width = screen.width;
          var height = screen.height;
      </script>";  
$width_screen = "<script>width</script>";
$height_screen = "<script>height</script>";