update statement doesn't do anything to my table

I get no errors and all my print statements are telling me i'm doing a good job, yet my field under the desired column stay the same.

what is happening (or rather not happening)?

if ($current && $new) {// the validation code is left out


        $sqlTekst = "UPDATE gebruikers SET pasW  = ". $_POST[nieuwPasw] ." WHERE gebruikersNr = ". $_POST[gebruikersNr];
        print 'huidig POST gebruikersNr = $_POST[gebruikersNr]';

// server koppeling
        $db = mysql_connect("localhost", "internet", "geheim");
// database koppeling
        mysql_select_db("site_met_aanmelden", $db)
 开发者_如何学Go               or die("Kan database gebruikers niet openen".mysql_error());        
        $resultaat = mysql_query($sqlTekst, $db);
        }   

Did you 'commit' the database changes?

add error handling for the actual query execution
take care of possible sql injections
check affected rows to see whether your WHERE-condition works or not
format and indent your sql statement and you'll get a slightly more meaningful error message
print your sql statement so you can check all of the parameters use $array['string'] instead of $array[string]

if ($current && $new) { // the validation code is left out
  $db = mysql_connect("localhost", "internet", "geheim") or die("connect failed. ".mysql_error());
  mysql_select_db("site_met_aanmelden", $db) or die("Kan database gebruikers niet openen".mysql_error());

  $sql_pasW = mysql_real_escape_string($_POST['nieuwPasw'], $db);
  $sql_gebruikersNr = mysql_real_escape_string($_POST['gebruikersNr'], $db);

  $sqlTekst = sprintf("
    UPDATE
      gebruikers
    SET
      pasW='%s'
    WHERE
      gebruikersNr ='%s'
    ", $sql_pasW, $sql_gebruikersNr
  );
  echo 'Debug '.__FILE__.__LINE__.': ', $sqlTekst, "\n"; 
  $resultaat = mysql_query($sqlTekst, $db) or die('query failed: '.mysql_error());

  echo 'affected rows: ', mysql_affected_rows($db);
}

I would fix it as follows:

$sqlTekst = "UPDATE gebruikers SET pasW  = '". $_POST["nieuwPasw"] ."' WHERE gebruikersNr = ". $_POST["gebruikersNr"];

Assuming your Nr refers to number which doesn't need quotes in the select.

I assume at least of your $_POST['nieuwPasw'] or $_POST['gebruikersNr'] is string and you don't enclose it with ' in your query.

Your working query would be

$sqlTekst = "UPDATE gebruikers SET pasW  = '". $_POST['nieuwPasw'] ."' WHERE gebruikersNr = '". $_POST['gebruikersNr']."'";

Just replace a string.

you can also get information if there error emerged in your query by using mysql_error function.

Add

if (!$resultaat) {
echo mysql_error();
}

this would print error occured during query.

I'd also suggest to enclose arrays keys in apostrophs (double or singular - does not really matter).

Try it like that:

 $sqlTekst = "UPDATE gebruikers SET pasW  = '". $_POST[nieuwPasw] ."' WHERE gebruikersNr = '". $_POST[gebruikersNr]."'";