高手进 sql注入失败！
自己写的登陆，测试sql注入，在数据库测试可以，但在这老是失败，求指教！
login.php
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>登陆界面-SQl注入测试</title></head><body><form method="post" name="form1" action="loginCl.php"><table><tr><td>用户名：</td><td><input type="text" name="username"/></td></tr><tr><td>密 码：</td><td><input type="password" name="password"/></td></tr><tr><td><input type="submit" value="登陆"/></td><td><input type="reset" value="重置"/></td></tr></table></form></body></html>
<?php$username = $_REQUEST['username'];$password = $_REQUEST['password'];$conn=mysql_connect('localhost','root','315824');if(!$conn){ echo "数据库连接失败".mysql_error();}mysql_select_db('test',$conn);$sql="select * from users where username='$username' and password='$password'";$res=mysql_query($sql,$conn);if( mysql_num_rows($res) != 0 ){ header("location:manageCenter.php");}else{ echo "用户名或密码错误，<a href='login.php'>重新登陆</a>";}?><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>管理中心</title></head><body>欢迎管理员，管理中心！</body></html>