$(function(){
    $('#login').on('submit',function(e){
        e.preventDefault();
        var user = $('#username').val();
        var pass = $('#password').val();
        $.ajax({
            url:'confirm_login.php',
            type:'POST',
            data:{user:user, pass: pass},
            success : function(data){
                alert(data);
            }
        });
    });
});

<?php
      echo "outside";
   if(isset($_POST['user']) && isset($_POST['pass'])){
      echo "inside";
   }
?>