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Very Simple PHP   MySql Login Form Not Working?
<?php
mysql_connect("localhost", "username", "password");
mysql_select_db("dbname");
?>

<body>
<?php
if(isset($_POST['submit'])){
$username = $_POST['username'];
$password = $_POST['password'];

$result = mysql_query("SELECT * FROM users WHERE username='$username' AND password='$password'");
$num = mysql_num_rows($result);
if($num == 0){
echo "Bad login, go <a href='login.php'>back</a>.";
}else{
session_start();
$_SESSION['username'] = $username;
header("Location: admin.php");
}


}else{
?>


<form action='login.php' method='post'>
   Username: <input type='text' name='username' /><br />
   Password: <input type='password' name='password' /><br /><br />
   <input type='submit' value='Login' />
</form>
<?php
}
?>
</body>

if(isset($_POST['submit'])){

<input type='submit' name='submit' value='Login' />

mysql_real_escape_string($_POST['username']));
  mysql_real_escape_string($_POST['username']));

<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post">

<?php // some php scripting ?>

<!-- blank lines also counts as output -->
<body>
  <?php session_start(); header("What ever"); ?>
</body>

<?php session_start(); ... ?>