.on('success.form.fv', function(e) {
            // Prevent form submission
            e.preventDefault();

            // Get the form instance
            var $form = $(e.target);

            // Get the BootstrapValidator instance
            var fv = $form.data('formValidator');

            // Use Ajax to submit form data
            $.post($form.attr('action'), $form.serialize(), function(result) {
                console.log(result);
                if(result === 'SIGN_IN') {
                   location.replace("form.php");
                } else {
                alert(result);
                }
            });

        });

<?

if(empty($_POST) === false) {
     $username = $_POST['username'];
     $password = $_POST['password'];



    if(empty($_POST['username'])===true || empty($_POST['password'])===true) {
        echo 'You need to enter email and password';
    }else if($db->user_exists($username) === false) {
        echo 'We can't find the user. Have you registered?';
    }else {

        $login = $db->login($username,$password);
        if($login===false) {
            echo 'Invalid password.';

        }else{
            $_SESSION['user_id'] = $login;
             echo 'SIGN_IN';
        exit;
        }
    }
}

?>