A note on PHP mysql_real_escape_string_PHP tutorial-PHP Tutorial-php.cn

A note on PHP mysql_real_escape_string_PHP tutorial

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Release： 2016-07-13 10:33:47

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The mysql_real_escape_string() function escapes special characters in strings used in SQL statements.

The following characters are affected:

If successful, the function returns the escaped string. If failed, returns false.

Usage is mysql_real_escape_string(string,connection)

Parameter string, required. Specifies the string to be escaped.
Parameter connection, optional. Specifies the MySQL connection. If not specified, the previous connection is used.

This function escapes special characters in a string and takes into account the current character set of the connection, so it is safe to use with mysql_query(). Use this function to prevent database attacks.

Simple use of mysql_real_escape_string()

<?php
$con = mysql_connect("localhost", "hello", "321");
if (!$con)
{
	die('Could not connect: ' . mysql_error());
}
// 获得用户名和密码的代码
// 转义用户名和密码，以便在 SQL 中使用
$user = mysql_real_escape_string($user);
$pwd = mysql_real_escape_string($pwd);
$sql = "SELECT * FROM users WHERE
user='" . $user . "' AND password='" . $pwd . "'"
// 更多代码
mysql_close($con);
?>

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SQL injection situation (mysql_real_escape_string() is not used)

Database attack. This example shows what happens if we don't apply the mysql_real_escape_string() function to the username and password:

<?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';
$_POST['pwd'] = "' OR ''='";
// 一些代码...
mysql_close($con);
?>

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Then the SQL query will look like this:

SELECT * FROM users
WHERE user='john' AND password='' OR ''=''

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This means that any user can log in without entering a valid password.

Correct ways to prevent database attacks:

<?php
function check_input($value)
{
	// 去除斜杠
	if (get_magic_quotes_gpc())
  	{
  		$value = stripslashes($value);
  	}
	// 如果不是数字则加引号
	//if (!is_numeric($value))
  	//{
  		$value = mysql_real_escape_string($value);
  	//}
	return $value;
}
$con = mysql_connect("localhost", "hello", "321");
if (!$con)
{
	die('Could not connect: ' . mysql_error());
}
// 进行安全的 SQL
$user = check_input($_POST['user']);
$pwd = check_input($_POST['pwd']);
$sql = "SELECT * FROM users WHERE
user=$user AND password=$pwd";
mysql_query($sql);
mysql_close($con);
?>

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Some experience

mysql_escape_string() also has a similar effect. Regarding these two functions, I remember that when I used the mysql_real_escape_string() function, the program always made errors, and I didn’t know the cause of the errors. Later, when I changed to the mysql_escape_string() function, it worked. Searching the manual, I found that when using mysql_real_escape_string(), a database connection must be established first, otherwise an error will be reported -___-|||

mysql_real_escape_string() calls MySQL's library function mysql_escape_string, which prepends backslashes to the following characters: NULL, x00, n, r, , ', " and x1a

mysql_escape_string() does not escape % and _.

This function is identical to mysql_real_escape_string() except that mysql_real_escape_string() takes a connection handler and escapes the string according to the current character set. mysql_escape_string() does not take a connection argument and does not respect the current charset setting.

<?php
// Connect
$link = mysql_connect('mysql_host', 'mysql_user', 'mysql_password')
 OR die(mysql_error());
// Query
$query = sprintf("SELECT * FROM users WHERE user='%s' AND password='%s'",
 mysql_real_escape_string($user),
 mysql_real_escape_string($password));
?>

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