openssl php RSA密钥生成错误，无法加密

$res = openssl_pkey_new();
@openssl_pkey_export($res,$pri);
@$d= openssl_pkey_get_details($res);
$pub = $d['key'];
var_dump($pub); //为什么会输出空？
$res = openssl_pkey_get_private($pri);
if (@openssl_sign('hello', $out, $res))
{
echo $out."
".base64_encode($out);
}
$sig = $out;
$sig = base64_decode($sig);
$res = openssl_pkey_get_public($pub);
if (@openssl_verify('hello', $sig, $res) === 1)
{
echo "ok";
}else{
echo "flase";
}

http://segmentfault.com/q/1010000002500045
http://blog.csdn.net/jom_ch/article/details/9303617