最近我更新了PHP版本，使用mysql_connect()函數會報錯：
PHP Warning:  mysql_connect(): mysqlnd cannot connect to MySQL 4.1+ using the old insecure authentication. Please use an administration tool to reset your password with the command SET PASSWORD = PASSWord('your_existing_password'). This will store a new, and more secure, hash value in mysql.user. If this user is used in other scripts executed by PHP 5.2 or earlier you might need to remove the old-passwords flag from your my.cnf file in /root/test.php on line 3

原因：
MySQL 5.0 為了和4.1之前的版本兼容，用戶密碼（mysql數據庫的user表password字段）保存的是密碼的16位hash值，而PHP5.3后的 mysql驅動使用41位的密碼hash值進行用戶驗證，所以需要讓mysql中保存用戶密碼的41位hash值，但配置文件中 old_passwords=1的話，password()函數只能返回16位hash值。

解決方法

1. SET old_passwords = 0;

2. update mysql.user SET PASSWORD = PASSWORD( 'phpddt.com' ) where User = 'root' LIMIT 1 ;

3. select LENGTH( PASSWORD ) from mysql.user where User = 'root';

4. FLUSH PRIVILEGES ;