Why Wordpress plugin url ajax doesn't work?

I need to send information to the plugin folder via ajax. Wordpress has a function for the link "plugin_dir_url (__ FILE__)" which works properly but if I put it in ajax it doesn't work.

Function: plugin_dir_url (__ FILE__) = http: // localhost / aa / wp-content / plugins / pluginfolder /

This works normally:

var xmlhttp = new XMLHttpRequest();
        xmlhttp.onreadystatechange = function() {
            if (this.readyState == 4  this.status == 200) {

            }
        };
        xmlhttp.open("GET", "http://localhost/aa/wp-content/plugins/pluginfolder/wp-getinfo.php?"SENDINFO=sendinfo, true);
        xmlhttp.send();

This does not work (php inside url):

var xmlhttp = new XMLHttpRequest();
        xmlhttp.onreadystatechange = function() {
            if (this.readyState == 4  this.status == 200) {

            }
        };
        xmlhttp.open("GET", ""+?php echo plugin_dir_url(__FILE__) .?+" wp-save.php?"SENDINFO=sendinfo, true);
        xmlhttp.send();

This does not work (php url as variable):

var urlwp = ?php echo plugin_dir_url(__FILE__) . 'wp-getinfo.php?' ?;

var xmlhttp = new XMLHttpRequest();
        xmlhttp.onreadystatechange = function() {
            if (this.readyState == 4  this.status == 200) {

            }
        };
        xmlhttp.open("GET", ""+urlwp+"SENDINFO=sendinfo, true);
        xmlhttp.send();