if($l = @mysql_connect('localhost', 'root', '123')) or die('数据库连接失败');
mysql_select_db('test');
mysql_set_charset('utf8');
$sql = 'select * from test where username = "$username" and password = "$password"';
$res = mysql_query($sql);
if(mysql_num_rows($res)){
header('Location:./home.php');
}else{
die('输入有误');
}

$sql = 'select * from test where username = "***" and password = "***" or 1 = "1"';

$sql = 'select * from test where username ="***" union select * from users/* and password = "***"';

$sql = "select * from test where username = $username and password = $password";

/* 过滤所有GET过来变量 */
foreach ($_GET as $get_key=>$get_var)
{
if (is_numeric($get_var)) {
$get[strtolower($get_key)] = get_int($get_var);
} else {
$get[strtolower($get_key)] = get_str($get_var);
}
}
/* 过滤所有POST过来的变量 */
foreach ($_POST as $post_key=>$post_var)
{
if (is_numeric($post_var)) {
$post[strtolower($post_key)] = get_int($post_var);
} else {
$post[strtolower($post_key)] = get_str($post_var);
}
}
/* 过滤函数 */
//整型过滤函数
function get_int($number)
{
return intval($number);
}
//字符串型过滤函数
function get_str($string)
{
if (!get_magic_quotes_gpc()) {
return addslashes($string);
}
return $string;
}

<?php  
function post_check($post) 
{ 
if (!get_magic_quotes_gpc()) // 判断magic_quotes_gpc是否为打开 
{ 
$post = addslashes($post); // 进行magic_quotes_gpc没有打开的情况对提交数据的过滤 
} 
$post = str_replace("_", "＼_", $post); // 把 '_'过滤掉 
$post = str_replace("%", "＼%", $post); // 把' % '过滤掉 
$post = nl2br($post); // 回车转换 
$post= htmlspecialchars($post); // html标记转换 
return $post; 
} 
?>