<?php
$con = mysql_connect("localhost", "hello", "321");
if (!$con)
  {
  die('Could not connect: ' . mysql_error());
  }

$sql = "SELECT * FROM users
WHERE user='{$_POST['user']}'
AND password='{$_POST['pwd']}'";
mysql_query($sql);

// 不检查用户名和密码
// 可以是用户输入的任何内容，比如：
$_POST['user'] = 'john';
$_POST['pwd'] = "' OR ''='";


mysql_close($con);
?>

 1 <?php
 2 function check_input($value)
 3 {
 4 // 去除斜杠
 5 if (get_magic_quotes_gpc())
 6   {
 7   $value = stripslashes($value);
 8   }
 9 // 如果不是数字则加引号
10 if (!is_numeric($value))
11   {
12   $value = "'" . mysql_real_escape_string($value) . "'";
13   }
14 return $value;
15 }
16 
17 $con = mysql_connect("localhost", "hello", "321");
18 if (!$con)
19   {
20   die('Could not connect: ' . mysql_error());
21   }
22 
23 // 进行安全的 SQL
24 $user = check_input($_POST['user']);
25 $pwd = check_input($_POST['pwd']);
26 $sql = "SELECT * FROM users WHERE
27 user=$user AND password=$pwd";
28 
29 mysql_query($sql);
30 
31 mysql_close($con);
32 ?>

 1 <?php
 2 function check_input($value)
 3 {
 4 // 去除斜杠
 5 if (get_magic_quotes_gpc())
 6   {
 7   $value = stripslashes($value);
 8   }
 9 
10 //过滤掉html标签
11 $value = strip_tags($value);
12 
13 // 如果不是数字则加引号
14 if (!is_numeric($value))
15   {
16   $value = "'" . mysql_real_escape_string($value) . "'";
17   }
18 return $value;
19 }
20 
21 $con = mysql_connect("localhost", "hello", "321");
22 if (!$con)
23   {
24   die('Could not connect: ' . mysql_error());
25   }
26 
27 // 进行安全的 SQL
28 $user = check_input($_POST['user']);
29 $pwd = check_input($_POST['pwd']);
30 $sql = "SELECT * FROM users WHERE
31 user=$user AND password=$pwd";
32 
33 mysql_query($sql);
34 
35 mysql_close($con);
36 ?>