Question

Recently, for no ‘apparent’ reason, my login form started to redirect to my website’s 404 page, and I don’t see the cause anywhere.

 <?php echo '<form action="http://' . $_SERVER['SERVER_NAME'] . '/login" method="post">'; ?>
 <li><input type="text" name="username" placeholder="Username"></li>
 <li><input type="password" name="password" placeholder="Password"></li>
 <li><input type="submit" value="Login"></li>
 </form>

Yet for some reason, it still redirects to the 404 page, when <?php echo'http://' . $_SERVER['SERVER_NAME'] . '/login is the exact same page. Apart from when the user is successfully logged in, there is no other redirecting, which goes to a different page. I just don’t see what’s causing this, can anyone else work out why?

EDIT:
My .htaccess is

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /index.php?page=$1

Authentication code:

<?php
if(empty($_POST) === false) {
   $username = $_POST['username'];
   $password = $_POST['password'];

   if(empty($username) === true || empty($password) === true) {
      $errors[] = 'You need to enter a username and password.';
   } else if (user_exists($username) === false) {
      $errors[] = 'We can\'t find that username. Have you registered?';
   } else if (user_active($username) === false) {
      $errors[] = 'You haven\'t activated your account!';
   } else if(strlen($password) > 32 ) {
        $errors[] = 'Password to long.';
   }
     $login = login($username, $password);
     if($login === false){
       $errors[] = 'That username/password combination is incorrect.';
     }else{
        $_SESSION['user_id'] = $login;
        header('Location: ' . $_SERVER['SERVER_NAME'] . '/?msg=1');
        exit();

     }
   }
if(empty($errors)===false){
?>
    <h1>We tried to log you in, but....</h1>
<?php
     print_r($errors);
} ?>

Answer 1

Can you leave out the $_SERVER[] arrays and use static url, just for debugging?
like:

<form method="POST" action="url.php">
<li><input type="text" name="username" placeholder="Username"></li>
<li><input type="password" name="password" placeholder="Password"></li>
<li><input type="submit" value="Login" name="submit"></li>
</form>

and:

 header('Location: url.php?msg=1');

If it is giving you a 404 Error, it must be either from this or your .htaccess
Debug it this way and let us know.