<form action="" method="post">
<?php

 $query = "Select * from rex_com_user order by name";

   $result = mysql_query($query);

   echo "<select name='name'>";

  while ($data = mysql_fetch_array($result))
   {
     echo "<option name=\"ID\" value='" . $data["name"] . " " . $data["firstname"] . " '>";
     echo $data["firstname"] . "</option>";
   }

  echo "</select>";
  
 ?>
<input type="submit" value="Suche starten" name="abschicken">
</form> 
<?php
$numOpt = 2;
$found = false;

if (isset($_POST['name']) && $_POST['name'] != ''){
echo '<p>ich guck nun in die datenbank unter id: '.$_POST['name'].'</p>';
$found = true;
}
?>

SQL Abfrage: finde alles, wenn ID = ID ist
Schleife
  Ergebnisse abholen
  Daten in korrektem HTML ausgeben
/Schleife