How to load key in RSA using phpseclib

Question

I am using the phpseclib3 library

use phpseclib3\\Crypt\\PublicKeyLoader;
use phpseclib3\\Crypt\\RSA;

$key = PublicKeyLoader::loadPrivateKey($private_key, $private_key_password);
return RSA::createKey()->loadPrivateKey($key, $private_key_password)->withHash('sha256')->withMGFHash('sha256')->withSaltLength(32)->sign($signing_string);

It is showing

error : load() should not be called from final classes (phpseclib3\\Crypt\\RSA\\PrivateKey)

I want to pass my Key to RSA to generate signing string.