if($select=="bitte auswählen" OR empty($select))
{
echo "Bitte eine Auswahl treffen";
}
else {
//anweisung
}

<form action="<?=$PHP_SELF."?".SID."&s=n_send_alt2"?>" method="post">
<select name="nr" class="selector">
<input type="hidden" name="sent" value="true">
<option class="selector"><?=$lang[choose]?></option>
<option class="selector">--------------------------</option>
<?

include($other."/mysql.php");
$dbres=@mysql_query("select subject, id from ".$cfg[dbpre]."content ORDER BY id DESC") or error(mysql_error());
@mysql_close($conn);

if($error=="") {
       while($row=mysql_fetch_array($dbres,MYSQL_ASSOC)) {
             echo "<option value=\"$row[id]\"class=\"selector\">".stripslashes($row[subject])."</option>";                       
       }
       mysql_free_result($dbres);
} else {
      echo "</select>


".$lang[follErr].": ".$error;
}

?>

</select> <input type="submit" class="butt" value="Weiter">
</form>

if($nr=="?was da so ausgeben wird?" OR $nr=="--------------------------" OR empty($nr))
{
echo "bitte auswählen";
}
else {
echo "inseide";
}

if($nr=="NON" OR empty($nr))
{
echo "bitte auswählen";
}
else {
echo "inseide";
}