求大神翻译

$link=mysql_connect("localhost", "root", "root");
mysql_select_db("zhucedenglu",$link);
session_start();
$username=$_POST['username'];
$passowrd=$_POST['password'];
if ($username && $passowrd){
$sql = "SELECT * FROM user WHERE username = '$username' and password='$passowrd'";
$res = mysql_query($sql);
$rows=mysql_num_rows($res);
if($rows){
$_SESSION['username']=$username;
echo"<script>alert('登录成功');location.href='index.php'</script>";
exit;
}
echo "<script language=javascript>alert('用户名或密码错误');history.back();</script>";
}
else
{
echo "<script language=javascript>alert('用户名或密码不能为空');history.back();</script>";
}
?>
以上代码每句的意思作用