PHP中的加密函数输出意外字符

I use simple_encrypt to store some sensitive information in the cookie. When I try to decrypt it with simple_decrypt, it doesn't give the same string. When I try to output that string after simple_decrypt I get chars with symbols like �. What's wrong?

$salt ='sososo222xxxXXsder3FVRE';
function simple_encrypt($text)
{
    global $salt;
    return mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $salt, $text, MCRYPT_MODE_ECB);
}

function simple_decrypt($text)
{
    global $salt;
    return mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $salt, $text, MCRYPT_MODE_ECB);
}

EDIT: I use setcookie("rt", simple_encrypt($a['rt']), time()+(3600 * 24 * 365)); to store cookie. I also put echo simple_encrypt($a['rt']); before setcooking, and got the different value that it is in the cookie. That's just amazing...

Try this:

function simple_encrypt($text)
{
    global $salt;
    return base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $salt, $text, MCRYPT_MODE_ECB));
}

function simple_decrypt($text)
{
    global $salt;
    return base64_decode(mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $salt, $text, MCRYPT_MODE_ECB));
}