我前不久也在倒腾这些，不过今天已经搞定了，完全与.net中的兼容

<?php
class Crypt3Des
{
private $key = "";
private $iv = "";
/**
* 构造，传递二个已经进行base64_encode的KEY与IV
*
* @param string $key
* @param string $iv
*/
function __construct ($key, $iv)
{
if (empty($key) || empty($iv)) {
echo 'key and iv is not valid';
exit();
}
$this->key = $key;
$this->iv = $iv;
}
/**
*加密
* @param <type> $value
* @return <type>
*/
public function encrypt ($value)
{
$td = mcrypt_module_open(MCRYPT_3DES, '', MCRYPT_MODE_CBC, '');
$iv = base64_decode($this->iv);
$value = $this->PaddingPKCS7($value);
$key = base64_decode($this->key);
mcrypt_generic_init($td, $key, $iv);
$ret = base64_encode(mcrypt_generic($td, $value));
mcrypt_generic_deinit($td);
mcrypt_module_close($td);
return $ret;
}
/**
*解密
* @param <type> $value
* @return <type>
*/
public function decrypt ($value)
{
$td = mcrypt_module_open(MCRYPT_3DES, '', MCRYPT_MODE_CBC, '');
$iv = base64_decode($this->iv);
$key = base64_decode($this->key);
mcrypt_generic_init($td, $key, $iv);
$ret = trim(mdecrypt_generic($td, base64_decode($value)));
$ret = $this->UnPaddingPKCS7($ret);
mcrypt_generic_deinit($td);
mcrypt_module_close($td);
return $ret;
}
private function PaddingPKCS7 ($data)
{
$block_size = mcrypt_get_block_size('tripledes', 'cbc');
$padding_char = $block_size - (strlen($data) % $block_size);
$data .= str_repeat(chr($padding_char), $padding_char);
return $data;
}
private function UnPaddingPKCS7 ($text)
{
$pad = ord($text{strlen($text) - 1});
if ($pad > strlen($text)) {
return false;
}
if (strspn($text, chr($pad), strlen($text) - $pad) != $pad) {
return false;
}
return substr($text, 0, - 1 * $pad);
}
}
?>

直接拿去用吧。