Drop Down List Issue In PHP

**1st File: DropDown.php**
<!DOCTYPE html>
<html>
<body>

  <?php

					$servername = "localhost";
					$username = "root";
					$password = "";
					$dbname = "movies";

					// Create connection
					$conn = new mysqli($servername, $username, $password, $dbname);

					// Check connection
					if ($conn->connect_error)
					{
						die("Connection failed: " . $conn->connect_error);
					}

				echo "<form method=\"post\" action = \"DropDownList.php\">";


					$actlist = "SELECT AID, ANAME FROM actors";
					$aresult = mysqli_query($conn, $actlist);

				echo "<select id=\"dd\" name=\"ddl\">";

			if (mysqli_num_rows($aresult) > 0)
			{

				echo "<option value = \" \" selected>ACTORS</option>";

					while($ares = mysqli_fetch_assoc($aresult))
					{

							$act = $ares["ANAME"];
							$aid = $ares["AID"];

					   echo "<option value = ".$aid.">".$aid."&nbsp;".$act. "</option>";

	//Below you can see the session variable, I used that to pass $aid of selected item from this file to another php file, //though the value got transferred but only the last item in the list is getting transferred even if I select the other item. //That's the problem I am facing
						/*session_start();
						$_SESSION['seVar'] = $aid;
						session_write_close();
						*/

					}
				}

				else
					{
						echo "No rows";
					}
						echo "</select>";
						echo $aid;
						echo "<input type=\"submit\" value=\"Submit\"></input>";
						echo "</form>";
				?>

	</body>
</html>

**2nd File: DropDownList.php**
<?php

$servername = "localhost";
$username = "root";
$password = "";
$dbname = "movies";
$onclk =$_POST["$aid"];


if(isset($onclk))
{
	echo $onclk; //Here I tried to receive the selected value from DropDown.php file, but unable to do so.
}


/*/ Create connection
$conn = new mysqli($servername, $username, $password, $dbname);

// Check connection
if ($conn->connect_error)
{
    die("Connection failed: " . $conn->connect_error);
}


$actlist = "SELECT AID FROM actors";
$aresult = mysqli_query($conn, $actlist);


 session_start();


  $x=0;


  $act = $_SESSION['seVar'];

   echo $act;         //It is displaying the last item of the list, doesn't matter which item I select from the drop down list.

session_write_close();


mysqli_close($conn);
*/

?>