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

<!--

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-->$username = $_REQUEST['username'];
$password = $_REQUEST['password'];
$sql="select * from users where username='$username' and password='$password'";

Copy after login

PHP code

<!--

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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