使用MySQL中的下拉变量进行排序

I'm new to PHP, and I have a simple table from a MySQL database. What I want to do is sort the table using the selected value from the drop down box, with the data from that row in the database.

Here is my code (selected is what I need for the drop down box):

$orderby = $_GET['orderby'];

if ($orderby == 'selected') { 
    $orderby_query = "WHERE timeStamp LIKE 'selected'";}
else { 
    $orderby_query = "";
}

$con = mysql_connect("xxxx","xxxx","xxxx");
$db = mysql_select_db("xxxx",$con);
$get = mysql_query("SELECT DISTINCT(timeStamp) FROM deliverys ORDER BY timeStamp DESC");
$timeStamp = '';

while($row = mysql_fetch_assoc($get))
{
    $timeStamp .= '<option value   ="'.$row['timeStamp'].'">'.$row['timeStamp'].'</option>';
}

<table width="100%" border="1">
<tr style="font-weight:bold;" valign="top" height="30px" bgcolor="#999999">
<td><form name="sorter0" action="index.php" method="get">
<label title="date">Date</label><br />
<select name="orderby" OnChange="document.sorter0.submit();"><br /><br />
<option>Sort by Date</option>
<option value=''>--?--</option><?php echo $timeStamp; ?></option>
</select>
</form></td>

I don't know if I'm adding the data to the drop down box correctly because, if I click, the site reloads and I can see it say:

/index.php?orderby=05-09-2015

If I use a "fixed" value like yes or no it works.