How to send an array to php and load again in html?

I've created a form with 3 select tag in my HTML, tried to create an array from it by jquery and send it to PHP. I want to load some data from my php and show in my html file. Here is my code

HTML code

<form name="myform">
    <select name="select1">
        <option value="">select1</option>
        <option value="11">value11</option>
        <option value="12">value12</option>
    </select>
    <select name="select2">
        <option value="">select2</option>
        <option value="21">value21</option>
        <option value="22">value22</option>
    </select>
    <select name="select3">
        <option value="">select3</option>
        <option value="31">value31</option>
        <option value="32">value32</option>
    </select>
</form>
<div id="mydiv"></div>

ajax code

$("form select").bind("change", function () {
        var myval = jQuery.param($('form select').serializeArray());
        alert(myval);
        $.ajax({
            type: "POST",
            url: "test.php",
            data: myval
        });
    });
    $("#mydiv").load("myfile.php");

In alert(myval) it alerts the value like this

select1=&select2=&select3=

this message changes every time I change on of my select tags. so I think this part of code has no problem!

PHP code

$postarr = $_POST['myval'];
var_dump($postarr)//gives me NULL
var_dump($_POST)//gives me array(0);
echo $postarr[0];

this message shows in my HTML(in #mydiv)

Notice: Undefined index: myval ...

my question is what is my problem and what index I must use in my php code.