Load database values to drop down

i have two drop down boxes and when user select District from 1st drop down box , the cityis in that district should load to the second drop down.
My code is below. But it is not working. Please tell me what is incorrect with it.

  <tr>
      <td width="111">
	<form action="test.php" method="post" name="test" >
		<select name="District"  class="defaultBlackText" onchange="this.form.submit();">
		<option <?php if ($_POST['District'] == 'G1') print 'selected '; ?> value="G1">G1</option>
		<option <?php if ($_POST['District'] == 'G2') print 'selected '; ?> value="G2">G2</option>
		<option <?php if ($_POST['District'] == 'G3') print 'selected '; ?> value="G3">Kurunegala</option>
		</select>  
	</form>  
	</td>
    <td width="15">&nbsp;</td>
    <td width="145">
	<form action="test.php" method="post" name="testl">
	<select name="city"  onchange="this.form.submit();">
     <option value="Select">Select</option>
    </select>
	</form>
	</td>
    <td width="138"><?php if (isset($_POST['District']))
		{
			echo $_POST['District'];
			$District=$_POST['District'];
			$q="SELECT * FROM xxx WHERE District='".$District."' ";
			$result=mysql_query($q) or die ("Could not execute query: $q.".mysql_error());
					while($row=mysql_fetch_array($result)){
					foreach( $row AS $key => $val ){
					$key = stripslashes( $val );
					}
						
					}
		}

?>	</td>
    <td width="158">
      </td>
    <td width="12">&nbsp;</td>
    <td width="190"></td>
	<td>&nbsp;</td>
  </tr>

PHP:

 

Your code is vulnerable.