Javascript用Ajax调用php函数

I need to show content on subfolders on my website. So far I have a SelectBox wich outputs the name of folders, I need (on the onchange event) to show files in that folder.

My select:

<select name="imgfolder" onchange="filelist(this.value)">   

Filelist is a JavaScript in the same page, which calls a php function in another file:

<script>    
    function filelist(selectedData)
    {       
        $.ajax({      
        type: 'POST',
        url: 'http://www.example.com/filelist.php',   

    dataType: 'json',    
    data: { folder: selectedData },
    success: function(response) 
    {        
    alert (response);

    }
    });

    }
</script>

And finally filelist.php page:

<?php

header('Content-Type: application/json');


  $folder = $_POST['folder'];
  echo json_encode($folder);

?>

At the moment it should simply return the name of the folder which is passed from JavaScript back to the caller and display an alert... But it doesn't work. What am I doing wrong?