将下拉列表值保存到php会话

Question

我已经创建了一个下拉列表，由mysql db中的数据填充，im试图将选定的值保存在会话变量中，但我不知道如何写这个（** * 是）？它是否需要张贴到服务器端前夕，虽然它的所有在PHP？

<?php 
$servername1 = "localhost"; 
$username1 = "root"; 
$password1 = ""; 
$dbname1 = "gpdb1"; 
$conn1 = new mysqli($servername1, $username1, $password1, $dbname1); 
if ($conn1->connect_error) { 
die("connection failed: " . $conn1->connect_error); 
} 
$sql2 = "SELECT DoctorID, Title, Surname FROM doctors"; 
$result2 = $conn1->query($sql2); 
echo "<select name='doctor' value=''><option>Select a Doctor</option>"; 
if ($result2->num_rows > 0) { 
foreach($result2 as $row2) 
{echo "<option value=".$row2['DoctorID'].">".$row2['Title']."  ".$row2['Surname']."</option>"; 
} 
}echo "</select>"; 
$_SESSION['selected']=***; 
if (isset($_SESSION['selected'])){ 
echo $SESSION['selected'];} 
?> 

谢谢:)