use javascript variable in php in jquery function

I want to use javascript variable in php function.. my code is as below

$(document).ready(function() { 
    $("#btn_url").on('click', function() {
        var video_val = $("#video_url").val(); alert(video_val);
        alert('<?php echo getYoutubeDurationV3("<script>document.write(video_val);</script>"); ?>');
    });
 }); 

I want to use video_val variable in php function.. how can i do that ?