Validering af login script.

Hej Eksperter.

Gider i lige kigge dette igennem og komme med ris/ros

<?php

$uri = "index.php";

trim($_POST['username']);
trim($_POST['password']);

if (empty($_POST['username'])) {
if (!empty($_POST['password'])) { 
  $uri .= "?password=".$_POST['password'];
  header("Location: $uri");
  exit;
}
else {;
  header("Location: $uri");
  exit;
}
}

elseif (empty($_POST['password'])) {
if (!empty($_POST['username'])) {
  $uri .= "?username=".$_POST['username'];
  header("Location: $uri");
  exit;
}
else {
  header("Location: $uri");
  exit;
}
}

else {
if ((!empty($_POST['username'])) && (!empty($_POST['password'])) && (ctype_alnum($_POST['username'])) && (ctype_alnum($_POST['password']))) {
  echo "Everything is okey, logging user in.";
}
else {
  $uri .= "?username=".$_POST['username']."&password=".$_POST['password'];
  header("Location: $uri");
}
}

?>