这段php代码怎么理解？

// user类中的方法：

public static function getByUsername($username) {
$u = new User();

        $query = sprintf('SELECT USER_ID, PASSWORD, EMAIL_ADDR, IS_ACTIVE ' .
            'FROM %sUSER WHERE USERNAME = "%s"',
            DB_TBL_PREFIX,
            mysql_real_escape_string($username, $GLOBALS['DB']));
        $result = mysql_query($query, $GLOBALS['DB']);

        if (mysql_num_rows($result))
        {
            $row = mysql_fetch_assoc($result);
            $u->username = $username;
            $u->password = $row['PASSWORD'];
            $u->emailAddr = $row['EMAIL_ADDR'];
            $u->isActive = $row['IS_ACTIVE'];
            $u->uid = $row['USER_ID'];
        }

mysql_free_result($result);

return $u;
}

// register.php页面中：

$user = User::getByUsername($_POST['username']);
 if ($user->userId)
 {
 $GLOBALS['TEMPLATE']['content'] = 'Sorry, that ' .
 'account already exists. Please try a ' .
 'different username.';
 $GLOBALS['TEMPLATE']['content'] .= $form;
 }
 else
 {
 // create an inactive user record
 $u = new User();
 $u->username = $_POST['username'];
 $u->password = $password;
 $u->emailAddr = $_POST['email'];
 $token = $u->setInactive();

$GLOBALS['TEMPLATE']['content'] = 'Thank you for ' .
 'registering. Be sure to verify your ' .
 'account by visiting <a href="verify.php?uid=' .
 $u->userId . '&token=' . $token . '">verify.php?uid=' .
 $u->userId . '&token=' . $token . '</a>';
 }

追问：$query = sprintf('SELECT USER_ID, PASSWORD, EMAIL_ADDR, IS_ACTIVE ' .
            'FROM %sUSER WHERE USERNAME = "%s"',
            DB_TBL_PREFIX,
$GLOBALS['TEMPLATE']['content'] = '<p><strong>Sorry, that ' .
                'account already exists.</strong></p> <p>Please try a ' .
                'different username.</p>';
            $GLOBALS['TEMPLATE']['content'] .= $form;