selecting option on menu A changes the options available in menu B

[newbeee]

if the category pull down menu is changed then the list in the topics pull down menu changes to show the topics that are in that category chosen. all data is got from the mysql database.

 

there maybe a lot of categories and topics in the furture, just in case that may help in creating a method to prevent the code needing changing again in the future, due to taking too long or something?

<select name="categoryid"><? // get category_id
$get_category_titles = mysql_query("SELECT * FROM `forum_category` WHERE `confirmed` = '1' ORDER BY `created_on` DESC");
$get_topic_titles = mysql_query("SELECT * FROM `forum_topics` WHERE `confirmed` = '1' ORDER BY `created_on` DESC");

for ($i = 0; $i < mysql_num_rows($get_category_titles); $i ++) { ?><option<?
if ($posts['categoryid'] == mysql_result($get_category_titles, $i, 'category_id')) {echo(" selected");} ?> value="<?=mysql_result($get_category_titles, $i, 'category_id');?>"><?=mysql_result($get_category_titles, $i, 'category');?></option><?
} ?></select>


<select name="topicid"><? // get topics
for ($i = 0; $i < mysql_num_rows($get_topic_titles); $i ++) {
?><option<? if ($posts['topicid'] == mysql_result($get_topic_titles, $i, 'topic_id')) {echo(" selected");}
?> value="<?=mysql_result($get_topic_titles, $i, 'topic_id');?>"><?=mysql_result($get_topic_titles, $i, 'topic');?></option><?
} ?></select>