<?php
$auth = false; //login status

if ($_SERVER['REQUEST_METHOD'] == 'POST')
{
    mysql_connect('127.0.0.1', 'root', '');
    mysql_select_db('test');

    //get data from db
    $query = 'SELECT password,salt FROM users WHERE username =' . mysql_escape_string($_POST['username']) . ' LIMIT 1';
    $data = (object) mysql_query($query);

    if ($data->password === md5($_POST['password'] . $data->salt))
    {
        $auth = true;
    }
}
?>

<?php if ($auth): ?>
    Login success
<?php else: ?>
        <form method="POST" action="/">
            <label>Username:</label>
            <input type="text" name="username" />
            <label>Password:</label>
            <input type="password" name="password" />
        </form>
<?php endif; ?>