在select标签上使用jquery获取php变量

I'm trying to get a string from a php file, here:

$company = Company::find_by_id($_GET['company_id']);
$string = $company->output_information();
$output = array('key' => $string);

echo json_encode($output);

The main file has the following html:

<select id="step_one_check">
 // some options
</select>
<div id="div_company_data">TBA</div>
<div id="step_two" style="display: none">Step two<div>

and the following jQuery:

$(document).ready(function(){
        $("#step_one_check").change(function(){
            $("#step_two").slideDown();
            $.ajax({
                url : '../includes/encode/company_information.php',
                type : 'GET',
                data : "company_id=" + $("#step_one_check").val(),
                dataType : 'json',
                success : function (result) {
                   alert("success");
                        $("#div_company_data").html(result.key);
                },
                error : function () {
                   alert("error");
                }
            });

        });

The jQuery script makes step two slides down, but does not alert or #div_company_data. Any idea?

----- UPDATE -----

I've resolved the problem, it's my php file. After I included the Company class, the code worked.