PHP学习：PHP加密3DES报错 Call to undefined function: mcrypt_module_open() 如何解决

<?php
/**
* 
* PHP版3DES加解密类
*
* 可与java的3DES(DESede)加密方式兼容
*
* @Author: Luo Hui (farmer.luo at gmail.com)
*
* @version: V0.1 2008.12.04
*
*/
class Crypt3Des
{ 
public $key = "01234567890123456789012345678912";
public $iv = "23456789"; //like java: private static byte[] myIV = { 50, 51, 52, 53, 54, 55, 56, 57 };
//加密
public function encrypt($input)
{
$input = $this->padding( $input );
$key = base64_decode($this->key);
$td = mcrypt_module_open( MCRYPT_3DES, '', MCRYPT_MODE_CBC, '');
//使用MCRYPT_3DES算法,cbc模式
mcrypt_generic_init($td, $key, $this->iv);
//初始处理
$data = mcrypt_generic($td, $input);
//加密
mcrypt_generic_deinit($td);
//结束
mcrypt_module_close($td);
$data = $this->removeBR(base64_encode($data));
return $data;
}
//解密
public function decrypt($encrypted)
{
$encrypted = base64_decode($encrypted);
$key = base64_decode($this->key);
$td = mcrypt_module_open( MCRYPT_3DES,'',MCRYPT_MODE_CBC,'');
//使用MCRYPT_3DES算法,cbc模式
mcrypt_generic_init($td, $key, $this->iv);
//初始处理
$decrypted = mdecrypt_generic($td, $encrypted);
//解密
mcrypt_generic_deinit($td);
//结束
mcrypt_module_close($td);
$decrypted = $this->removePadding($decrypted);
return $decrypted;
}
//填充暗码,填充至8的倍数
public function padding( $str )
{
$len = 8 - strlen( $str ) % 8;
for ( $i = 0; $i < $len; $i++ )
{
$str .= chr( 0 );
}
return $str ;
}
//删除填充符
public function removePadding( $str )
{
$len = strlen( $str );
$newstr = "";
$str = str_split($str);
for ($i = 0; $i < $len; $i++ )
{
if ($str[$i] != chr( 0 ))
{
$newstr .= $str[$i];
}
}
return $newstr;
}
//删除回车和换行
public function removeBR( $str ) 
{
$len = strlen( $str );
$newstr = "";
$str = str_split($str);
for ($i = 0; $i < $len; $i++ )
{
if ($str[$i] != '\n' and $str[$i] != '\r')
{
$newstr .= $str[$i];
}
}
return $newstr;
}
}
//test
$input = "1qaz2ws";
echo "plainText:" . $input."<br/>";
$crypt = new Crypt3Des();
echo "Encode:".$crypt->encrypt($input)."<br/>";
echo "Decode:".$crypt->decrypt($crypt->encrypt($input));
?>