how to get java script variable in php variable

First way =>
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 <script>
var x = document.write(""<?php echo $b; ?>) ;
alert(x);
</script>

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Second way =>
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<script>
var id = jQuery(this).attr("id");
        var comment = jQuery("#"+id).val();
        var dataString = "comment="+comment+"&user_id=<?=base64_encode(json_encode($_SESSION['user_id']))?>"+"&ans_id="+id;
jQuery.ajax({
                type: "POST",  
                url: "page/xyz.php",  
                data: dataString,  
                beforeSend:function(){
                  jQuery('#loader_pic').css("display","block"); 
                }, 
                success: function(data){ 
                    //alert(data);
                    jQuery('#loader_pic').css("display","none");
                    jQuery("#show_more_comment_"+id).append(data);
                    jQuery("#"+id).val('');
                },
                error:function(data){
                    jQuery('#loader_pic').css("display","none");
                    alert(data);
                  
                }       
            });
</script>
<a href="" onclick="abc(12)" />