最近我更新了PHP版本，使用mysql_connect()函数会报错：
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 为了和4.1之前的版本兼容，用户密码（mysql数据库的user表password字段）保存的是密码的16位hash值，而PHP5.3后的 mysql驱动使用41位的密码hash值进行用户验证，所以需要让mysql中保存用户密码的41位hash值，但配置文件中 old_passwords=1的话，password()函数只能返回16位hash值。

解决方法

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 ;