关于sql注入的请问

关于sql注入的请教
刚才看了这个帖子
http://topic.csdn.net/u/20121011/08/ed0d9538-0ed1-49c4-95c3-0fc1ef686aa2.html

他的登录代码是这样的：

PHP code

<!--Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/-->$username = $_REQUEST['username'];$password = $_REQUEST['password'];$sql="select * from users where username='$username' and password='$password'";

Copy after login

PHP code

<!--Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/-->$username = $_REQUEST['username'];$password = md5($_REQUEST['password']);//改了这行$sql="select * from users where username='$username' and password='$password'";

Copy after login