Dropdown reading array and onclick

Hello, i am all new to this kind of stuff and i didnt find any topic yet about it because i do not really know how to call this problem. So if there are any solutions yet i am sorry for opening a new topic.

I want a dropdown navigation, that read its content out of an array.

thats what i got so far and it works.

Code:

<?
  		echo '<SELECT name=bild>';
  		        
              foreach ($zahl_array as $key => $value)
              {
                echo '<OPTION value='.$value.'> '.$value.'';
              }
      echo '</select>'; 
      ?>

$zahl_array is reading the array content out of a .txt file.

Now the problem is that i want something to happen when you change/click on one link in the dropdown.

Above this code, there is a table with an randomly generated Image.

Code:

$zahl_array = file('zahl.txt');
$num = rand(0, count($zahl_array) - 1);
$zahl = $zahl_array[$num];

//In here <--> should be the if-clause.
//1. possiblity. Nothing is choosed so it stays random like this
...html...<img src="/images/<? echo $zahl; ?>.jpg">...html...
//2.possibility. A link is choosed out of the dropdown.
...html...<img src="/images/<? echo $choosed_zahl; ?>.jpg">...html...

Now if the user chooses one link in the dropdown it should change the Image above to the one that the user choosed.

Sry for my bad english i am from germany

 

I think you have to use AJAX to do this. Here's a very good library that might help you to accomplish this: http://jquery.com/