[Nexus]

function set_php_variable(value) {
    var xhr = new XMLHttpRequest();
    xhr.open('POST', '/listener.php', false);
    xhr.send('variable=' + encodeURIComponent(value));
}

var x1=10;
set_php_variable(x1);

$b = isset($_POST['variable']) ? htmlspecialchars($_POST['variable']) : null;

var_dump($b);