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？？？