A few questions for clarity:
1) Is the select box going to be generatde with PHP and SQL?
2) "on the same page" Does that mean you don't want to reload the page or you just want the results displayed at the same address?
Ah.. hell.. Here's some sample code to get ya' going:
//FOR THE SELECT BOX
$q = "SELECT id, name FROM table";
$r = mysql_query($q);
echo "<select name=\"myselectbox\">";
while($row = mysql_fetch_assoc($r))
{
echo "<option value=\"".$row['id']."\">".$row['name']."</option>";
}
echo "</select>";
//THE LOGIC TO DO THE WORK ON THE SAME PAGE
if(isset($_POST['submit']))
{
//the form was submitted..
//do some work with the data
}
else
{
//show th form with a button named 'submit'
}