php rsa加密 已有明文和公钥 只需加密

代码:
$encrypted = '';
$key = openssl_pkey_get_public($pubkey);
var_dump($key);
openssl_public_encrypt('a',$encrypted,$key);
echo $encrypted;

失败提示:
Warning: openssl_public_encrypt(): Don't know how to get public key from this private key in E:\WWW\php_20170720\api\bdy\api.php on line 131

Warning: openssl_public_encrypt(): key parameter is not a valid public key in E:\WWW\php_20170720\api\bdy\api.php on line 131

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