php字符串的加解密问题

$Str = 'This is a test string';
$Key = '9080T7&@_F98d799';
$IV = 'ABCEdfghiJ123*#$';

$EnStr = mcrypt_cbc(MCRYPT_RIJNDAEL_128,$Key,$Str,MCRYPT_ENCRYPT,$IV);

$DeStr = mcrypt_cbc(MCRYPT_RIJNDAEL_128,$Key,$EnStr,MCRYPT_DECRYPT,$IV);

echo $DeStr;


为什么输出的不是原字符串，而是原字符串后面跟着一些乱码字符？？？