PHP,the wrong result when use mycrypt_cfb function.

Question

here's the encrypt and decrypt function:

    function encrypt($key, $plain_text) {
    $plain_text = trim($plain_text);
    $iv = substr(md5($key), 0,mcrypt_get_iv_size (MCRYPT_CAST_256,MCRYPT_MODE_CFB));
    $c_t = mcrypt_cfb (MCRYPT_CAST_256, $key, $plain_text, MCRYPT_ENCRYPT, $iv);
    return trim(chop(base64_encode($c_t)));
}

function decrypt($key, $c_t) {
    $c_t =  trim(chop(base64_decode($c_t)));
    $iv = substr(md5($key), 0,mcrypt_get_iv_size (MCRYPT_CAST_256,MCRYPT_MODE_CFB));
    $p_t = mcrypt_cfb (MCRYPT_CAST_256, $key, $c_t, MCRYPT_DECRYPT, $iv);
    return trim(chop($p_t));
}

now:

$key = "11111"; $str = "1000181"; echo decrypt($key,encrypt($key,$str));

the result is 100018,missing the last "1".

why????