如何选择类别中的所有项目

This is the goal: The user needs to view the problems for a particular category. It is the oldest problem on top in the list. This is the code that I use but it has an error in it . When I click the button "Zoeken" nothing happens. The page just refreshes. Any help?

<form method="post" action="<?php echo $_SERVER["h5/PHP_SELF"] ?>">
Selecteer een categorie:<select name="categorieid">
<?php
$db=mysql_connect("localhost","*****","******") or die("Kan niet verbinden: ".mysql_error());
mysql_select_db("*****",$db);

    $sql = "SELECT * FROM Categorie";

    $resultaat  = mysql_query($sql); // voer SQL code uit
    while ($rij = mysql_fetch_array($resultaat)) {
        echo "<option value=\"";
        echo $rij["Categorie_ID"]."\">";
        echo $rij["Categorienaam"]."</option>
";
    }

mysql_close($db);
?>
//de tabel weergeven
<?php
if ($_POST["Zoek"]) {
$_POST["categorieid"];
$db=mysql_connect("localhost","********","********") or die("Kan niet verbinden: ".mysql_error());
mysql_select_db("**********",$db);

$sql = "SELECT * FROM Probleem";
if(isset($_POST["categorieid"])){
    $cat_id = $_POST["categorieid"];
    //sanitize $cat_id to reduce injection risk here
    $sql .= " WHERE Categorie_ID = $cat_id"; //use correct column name
}
$sql .= " ORDER BY Datum ASC";
$resultaat  = mysql_query($sql); // voer SQL code uit

echo "<table border=1>";
echo "<tr><td><b>Probleem_ID</b></td><td><b>Categorie</b></td><td><b>Datum</b></td><td><b>Omschrijving;</b></td><td><b>Gebruiker_ID</b></td><td><b>Oplossing</b></td></tr>"; // bovenste regel
if ($resultaat) {
    while ($rij = mysql_fetch_array($resultaat)) {

    echo "<tr>";
echo "<td>".$rij['Probleem_ID']."</td>";
echo "<td>".$rij['Categorie_ID']."</td>";
echo "<td>".$rij['Datum']."</td>";
echo "<td>".$rij['Probleem']."</td>";
echo "<td>".$rij['Gebruiker_ID']."</td>";
echo "<td>".$rij['Oplossing']."</td>";
echo "</tr>";
    }
}
echo "</table>";
mysql_close($db);
}
?>

<form name="form1" method="post" action="">
  <input type="submit" name="Zoek" id="Zoek" value="zoek">
</form>

As @Bonzo said:

Your "Zoeken" button should be in the same section as the rest of the form rather than in a different form.

PS. This question is not in section "Unanswered / No answers" anymore ;)