html forms, php, and and (refreshing?) pages

[DeMuro1]

here's my basic deal. I have a pulldown menu at the top of a page. I want to select an item on the pulldown and then have that open more fields below based on the selected item in the pulldown menu.

the only way I know how to do this is with an "if" block. I'm not sure though if that will automatically go to the if block.

if my html looks like this


<form action="pd_opt.php" method="get" name="myform">
What type of site addition is this?
<select name="pulldown">
<option value="1">option 1</option>
<option value="2">option 2</option>
</select><br>
</form>

and my php looks like this

PHP Code:

if ($pd_opt=1)
    {                    };
elseif ($pd_opt=2)
    {                    }; 


Now will this update the lower part of the mage immediately or do I need to add something that I'm not aware of. Also what do I put in my {braces} that allows me to open another page, another php file or in my case add php to the bottom portion of a page

Thanks. I'm really new to php and I'm sorry as this most likely is a very trivial question