jQuery $ .post问题（不向PHP发送变量）

I came across a little problem and struggling to solve it... Was wondering if you had any ideas... I have read A LOT of stackoverflow posts about the same issues, but still can't solve mine. I am workin in CodeIgniter.

I am trying to send the variable to PHP controller form the view using jQuery ($.post). Here is mine js code, that is embedded into the view:

$('#testbtn').on('click', function() {
        var orderValue = this.value;
        $.post('http://localhost/codeigniter/welcome/index', {val: 'myValue'}, function(data) {
            alert(data);
        });
    });

Here is the PHP code in the controller:

$a = isset($_POST['val']) ? $_POST['val'] : 'Not set yet.';
echo $a ;

Here is the button: <button id="testbtn" value="thevalue">button</button>

The button is clicked, I get the alert message with the variable and the html code of the whole page, but PHP echo doesn't change, still says: "Not set yet.".

Basically it seems to be very easy, but I can't still find the error...

Without diving into the code more, this would work.

HTML:

<button id="testbtn" value="thevalue">button</button>

Javascript:

$('#testbtn').on('click', function() {
        var orderValue = {"orderValue": $(this).val()};
        $.ajax({
        url: 'http://localhost/codeigniter/welcome/index',
        method: 'POST',
        data: orderValue,
        success: function(data) { alert(data); }
    });

PHP:

public function index() {
    $a = isset($_POST['orderValue']) ? $_POST['orderValue'] : 'Not set yet.';
    return a;
}

As stated in other comments though, this may not be the best practice.