/**
- PBKDF2 加密函数
- 参考标准
- @link https://www.ietf.org/rfc/rfc2898.txt
- php官方函数将在php5.5发布
- @see http://php.net/manual/en/function.hash-pbkdf2.php
- @example: pbkdf2("sha256", 'password', 'salt', 1, 20);
- result:120fb6cffcf8b32c43e7 (与php5.5内置的pbkdf2函数输出一至)
- @param string $algo The hash algorithm to use. Recommended: SHA256
- @param string $password The password to use for the derivation.
- @param string $salt The salt to use for the derivation.
- @param int $iterations The number of internal iterations to perform for the derivation.
- @param int $length The length of the derived key to output. If 0, the length of the supplied algorithm is used.
- @param boolean $raw_output When set to TRUE, outputs raw binary data. FALSE outputs lowercase hexits.
- @return string
*/