PHP login takes two attempts to work

[wopps93]

I've got a really simple login script using PHP's sessions to limit access, but I'm having a really peculiar issue. The login always fails on the first attempt, even with correct credentials, but second and subsequent attempts work with no issues. I'm really confused as to the cause, so any help would be appreciated.the login form, with non-relevant code removed (yes, I'm aware that storing the login stuff inside the PHP file as plain text is normally a bad idea, but it's not relevant in this case):\[code\]<?php$username = $_POST['username'];$password = $_POST['password'];$valid_user = "user";$valid_password = "password";if (($_POST['op'] == "ds") && ($username == $valid_user) && ($password == $valid_password)) { session_start(); session_register('valid'); $_SESSION['valid'] = 'yes'; header("Location: valid.php");}?><p>Username:<br><input type="text" name="username" size=15 maxlength=25></p><p>Password:</strong><br><input type="password" name="password" size=15 maxlength=25></p><input type="hidden" name="op" value="http://stackoverflow.com/questions/3747061/ds"><p><input type="submit" name="submit" value="http://stackoverflow.com/questions/3747061/login"></p>\[/code\]and every page that needs authenications has\[code\]require "auth.php"\[/code\]auth.php:\[code\]<?phpsession_start();if($_SESSION['valid'] != 'yes') { header("Location: login.php");}?>\[/code\]