dependable dropdown

Dag allemaal

Ik heb hier een script gemaakt waarbij ik informatie kan filteren tussen afdeling en subafdeling. Hij werkt goed, alleen wil er een dependable dropdown inbouwen maar weet niet hoe precies.
heb meerdere voorbeelden op internet gevonden, maar die konden mij niet helpen.
Hieronder mijn database indeling

afdeling

id
naam_afdeling

subafdeling

id
naam_subafdeling
afdeling_id

help_mij_zoeken.php

<?php 
require_once('class/zoeken.php'); //haal class op

startDB();  //functie startDB

//sql query voor het zoeken van trainingen
$sql = "SELECT DISTINCT train.id,train.naam_training AS Training, train.beschrijving AS Beschrijving, type_train.type_plaatje AS Plaatje
FROM training train

JOIN training_afdeling train_afd
	ON train_afd.training_id = train.id

JOIN afdeling afd
	ON afd.id = train_afd.afdeling_id

JOIN training_subafdeling train_subafd
	ON train_subafd.training_id = train.id

JOIN subafdeling subafd
	ON subafd.id = train_subafd.subafdeling_id	
	
JOIN training_type train_type
	ON train_type.training_id = train.id

JOIN type_training type_train
	ON type_train.id = train_type.type_id	
";
if (isset($_GET['srch_for'])){

	$getters = array();
	$queries = array();
	
	foreach($_GET as $key => $value) {
		$temp = is_array($value) ? $value : trim($value);
		if(!empty($temp)){
			list($key) = explode("-",$key);
			if (!in_array($key, $getters)){
				$getters[$key] = $value;
			}
		}
	}
	
	if (!empty($getters)){
	
		foreach($getters as $key => $value){
			${$key} = $value;
		switch($key){
			case 'srch_for':
			array_push($queries,"(train.naam_training LIKE '%$srch_for%' || train.beschrijving LIKE '%$srch_for%')"); //als de gezochte term overeenkomt met de waarde uit de database(alles)
			break;
			case 'zk_afdeling':
			array_push($queries, "train_afd.afdeling_id = $zk_afdeling"); //als de gezochte term overeenkomt met de waarde uit de database(afdeling)
			break;
			case 'zk_subafdeling':
			array_push($queries, "train_subafd.afdeling_id = $zk_subafdeling"); //als de gezochte term overeenkomt met de waarde uit de database(subafdeling)
			break;
			case 'zk_typ':
			array_push($queries, "train_type.type_id = $zk_typ"); //als de gezochte term overeenkomt met de waarde uit de database(type)
			break;
			}
		}
	}
	
	if (!empty($queries)) {
		
		$sql .= " WHERE ";
		$i=1;
		foreach($queries as $query){
			if ($i < count($queries)){
				$sql .= $query." AND ";
			} else {
				$sql .= $query;
			}
			$i++;
		}
	}
	$sql .= " ORDER BY train.naam_training ASC"; //einde query
}
$rs = mysql_query($sql) or die(mysql_error());

$rows = mysql_fetch_assoc($rs);
$tot_rows = mysql_num_rows($rs);
?>
<form id="msform" name="search_form" method="get" action="help_mij_zoeken_resultaat.php">             
        <div>                
                    <table border="0" cellspacing="0" cellpadding="0" class="tbl_search">
		<tr>
		
		<td><input type="hidden" name="srch_for" id="srch_for" class="f_fld" value="<?php getSticky(1,'srch_for'); ?>"></td>
		<th><label for="zk_afdeling">Afdeling:</table>
                    <?php getAfdeling(); ?><br>
	    </div>  
    <div>
      <label for="zk_subafdeling">Subafdeling:</th>
          <?php getSubafdeling(); ?>
    </div>    
    <input type="submit" id="btn" class="next action-button2" value="Zoeken" />
    </fieldset>
	<!-- einde pagina selecteer type training -->
    </form>
<?php mysql_free_result($rs); ?>

zoeken.php

<?php
require_once 'class/Singleton.php';

function startDB(){
	$connect = Singleton::getconnect();
}

function getAfdeling() {
	global $database;
	global $conndb;
	
	$sql = "SELECT * FROM afdeling ORDER BY naam_afdeling ASC";

	mysql_select_db($database);
	$rs = mysql_query($sql) or die(mysql_error());	
	$rows = mysql_fetch_assoc($rs);
	$total_rows = mysql_num_rows($rs);
	if($total_rows > 0){
		echo "<select name=\"zk_afdeling\" id=\"zk_afdeling\">\n";
		echo "<option value=\"\">Elke afdeling&hellip;</option>\n";
		do {
			echo "<option value=\"".$rows['id']."\"";
			getSticky(2, 'zk_afdeling',$rows['id']);
			echo ">".$rows['naam_afdeling']."</optio>";
		} while($rows = mysql_fetch_assoc($rs));
		echo "</select>";
	}
	mysql_free_result($rs);
}

function getSubafdeling() {
	global $database;
	global $conndb;
	
	$sql = "SELECT * FROM subafdeling ORDER BY naam_subafdeling ASC";

	mysql_select_db($database);
	$rs = mysql_query($sql) or die(mysql_error());
	$rows = mysql_fetch_assoc($rs);
	$total_rows = mysql_num_rows($rs);
	if($total_rows > 0){
		echo "<select name=\"zk_subafdeling\" id=\"zk_subafdeling\">\n";
		echo "<option value=\"\">Elke subafdeling&hellip;</option>\n";
		do {
			echo "<option value=\"".$rows['id']."\"";
			getSticky(2, 'zk_subafdeling',$rows['id']);
			echo ">".$rows['naam_subafdeling']."</optio>";
		} while($rows = mysql_fetch_assoc($rs));
		echo "</select>";
	}
	mysql_free_result($rs);
}

function getSticky($case,$par,$value="",$initial=""){
switch($case) {
	case 1: //text field
	if (isset($_GET[$par]) && $_GET[$par] !=""){
		echo stripslashes($_GET[$par]);
	}
	break;
	case 2: //select dropdown menus
	if (isset($_GET[$par]) && $_GET[$par] == $value){
		echo " selected=\"selected\"";
	}
	break;
	case 3: //checkbox group
	if (isset($_GET[$par]) && $_GET[$par] !=""){
		echo " checked=\"checked\"";
	}
	case 4: //radio buttons
	if (isset($_GET[$par]) && $_GET[$par] == $value){
		echo " checked=\"checked\"";
	} else {
	 if ($initial !=""){
		echo " checked=\"checked\"";
	 }
	}
	break;
}
}
?>