看注释，啥也不说了，欢迎各种跨平台测试！

/** 
 * 常用对称加密算法类 
 * 支持密钥：64/128/256 bit（字节长度8/16/32） 
 * 支持算法：DES/AES（根据密钥长度自动匹配使用：DES:64bit AES:128/256bit） 
 * 支持模式：CBC/ECB/OFB/CFB 
 * 密文编码：base64字符串/十六进制字符串/二进制字符串流 
 * 填充方式: PKCS5Padding（DES） 
 * 
 * @author: linvo 
 * @version: 1.0.0 
 * @date: 2013/1/10 
 */  
class Xcrypt{  
  
    private $mcrypt;  
    private $key;  
    private $mode;  
    private $iv;  
    private $blocksize;  
  
    /** 
     * 构造函数 
     * 
     * @param string 密钥 
     * @param string 模式 
     * @param string 向量（"off":不使用 / "auto":自动 / 其他:指定值，长度同密钥） 
     */  
    public function __construct($key, $mode = 'cbc', $iv = "off"){  
        switch (strlen($key)){  
        case 8:  
            $this->mcrypt = MCRYPT_DES;  
            break;  
        case 16:  
            $this->mcrypt = MCRYPT_RIJNDAEL_128;  
            break;  
        case 32:  
            $this->mcrypt = MCRYPT_RIJNDAEL_256;  
            break;  
        default:  
            die("Key size must be 8/16/32");  
        }  
  
        $this->key = $key;  
  
        switch (strtolower($mode)){  
        case 'ofb':  
            $this->mode = MCRYPT_MODE_OFB;  
            if ($iv == 'off') die('OFB must give a IV'); //OFB必须有向量  
            break;  
        case 'cfb':  
            $this->mode = MCRYPT_MODE_CFB;  
            if ($iv == 'off') die('CFB must give a IV'); //CFB必须有向量  
            break;  
        case 'ecb':  
            $this->mode = MCRYPT_MODE_ECB;  
            $iv = 'off'; //ECB不需要向量  
            break;  
        case 'cbc':  
        default:  
            $this->mode = MCRYPT_MODE_CBC;  
        }  
  
        switch (strtolower($iv)){  
        case "off":  
            $this->iv = null;  
            break;  
        case "auto":  
            $source = PHP_OS=='WINNT' ? MCRYPT_RAND : MCRYPT_DEV_RANDOM;  
            $this->iv = mcrypt_create_iv(mcrypt_get_block_size($this->mcrypt, $this->mode), $source);  
            break;  
        default:  
            $this->iv = $iv;  
        }  
  
     
    }  
  
  
    /** 
     * 获取向量值 
     * @param string 向量值编码（base64/hex/bin） 
     * @return string 向量值 
     */  
    public function getIV($code = 'base64'){  
        switch ($code){  
        case 'base64':  
            $ret = base64_encode($this->iv);  
            break;  
        case 'hex':  
            $ret = bin2hex($this->iv);  
            break;  
        case 'bin':  
        default:  
            $ret = $this->iv;  
        }  
        return $ret;  
    }  
  
  
    /** 
     * 加密 
     * @param string 明文 
     * @param string 密文编码（base64/hex/bin） 
     * @return string 密文 
     */  
    public function encrypt($str, $code = 'base64'){  
        if ($this->mcrypt == MCRYPT_DES) $str = $this->_pkcs5Pad($str);  
  
        if (isset($this->iv)) {  
            $result = mcrypt_encrypt($this->mcrypt, $this->key, $str, $this->mode, $this->iv);    
        } else {  
            @$result = mcrypt_encrypt($this->mcrypt, $this->key, $str, $this->mode);    
        }  
  
        switch ($code){  
        case 'base64':  
            $ret = base64_encode($result);  
            break;  
        case 'hex':  
            $ret = bin2hex($result);  
            break;  
        case 'bin':  
        default:  
            $ret = $result;  
        }  
   
        return $ret;  
   
    }  
  
    /** 
     * 解密  
     * @param string 密文 
     * @param string 密文编码（base64/hex/bin） 
     * @return string 明文 
     */  
    public function decrypt($str, $code = "base64"){      
        $ret = false;  
  
        switch ($code){  
        case 'base64':  
            $str = base64_decode($str);  
            break;  
        case 'hex':  
            $str = $this->_hex2bin($str);  
            break;  
        case 'bin':  
        default:  
        }  
  
        if ($str !== false){  
            if (isset($this->iv)) {  
                $ret = mcrypt_decrypt($this->mcrypt, $this->key, $str, $this->mode, $this->iv);    
            } else {  
                @$ret = mcrypt_decrypt($this->mcrypt, $this->key, $str, $this->mode);    
            }  
            if ($this->mcrypt == MCRYPT_DES) $ret = $this->_pkcs5Unpad($ret);  
            $ret = trim($ret);  
        }  
  
        return $ret;   
    }   
  
  
  
    private function _pkcs5Pad($text){  
        $this->blocksize = mcrypt_get_block_size($this->mcrypt, $this->mode);    
        $pad = $this->blocksize - (strlen($text) % $this->blocksize);  
        return $text . str_repeat(chr($pad), $pad);  
    }  
  
    private function _pkcs5Unpad($text){  
        $pad = ord($text{strlen($text) - 1});  
        if ($pad > strlen($text)) return false;  
        if (strspn($text, chr($pad), strlen($text) - $pad) != $pad) return false;  
        $ret = substr($text, 0, -1 * $pad);  
        return $ret;  
    }  
  
    private function _hex2bin($hex = false){  
        $ret = $hex !== false && preg_match('/^[0-9a-fA-F]+$/i', $hex) ? pack("H*", $hex) : false;      
        return $ret;  
    }  
  
  
  
}  

给个小demo吧

<?php  
header('Content-Type:text/html;Charset=utf-8;');  
  
include "xcrypt.php";  
  
echo '<pre>';  
//////////////////////////////////////  
$a = isset($_GET['a']) ? $_GET['a'] : '测试123';  
  
//密钥  
$key = '12345678123456781234567812345678'; //256 bit  
$key = '1234567812345678'; //128 bit  
$key = '12345678'; //64 bit  
  
//设置模式和IV  
$m = new Xcrypt($key, 'cbc', 'auto');  
  
//获取向量值  
echo '向量：';  
var_dump($m->getIV());  
  
//加密  
$b = $m->encrypt($a, 'base64');  
//解密  
$c = $m->decrypt($b, 'base64');  
  
echo '加密后：';  
var_dump($b);  
echo '解密后：';  
var_dump($c);  
  
  
/////////////////////////////////////////  
echo '</pre>';  

http://blog.csdn.net/linvo/article/details/8497056