<table>
   <tr>
      <td>Username:</td>
      <td><input type="text" name="username" maxlength="25" required placeholder="User"/>/td>
   </tr>
   <tr>
      <td>Password:</td>
      <td><input type="password" name="password" maxlength="25" required placeholder="************" /></td>
   </tr>
   <tr>
      <td colspan="2"><input type="submit" name="login" value="Log in" /></td>
   </tr>
</table>

if ( $_POST['login'] )
{

    $username = mysql_real_escape_string($_POST['username']);
    $password = mysql_real_escape_string($_POST['password']);
    $pas_hash = md5($password);

    $sql = mysql_query("SELECT * FROM users WHERE username='$username' AND password='$pas_hash' LIMIT 1");

    if( mysql_num_rows($sql) == 1 )
    {
        $row = mysql_fetch_array($sql);
        $_SESSION['id'] = $row['id'];
        header('/* YOUR MAIN PAGE */');
    }
    else
    {
        echo 'FAILED';
    }
}