PKCS8 concerns key formats - not signatures.
Also, I see you're using phpseclib to convert the key to pem and then using openssl. Why not just use phpseclib for everything? At that point you could use this example:
http://phpseclib.sourceforge.net/rsa/examples.html#sign,sign2
<?php
include('Crypt/RSA.php');
$rsa = new Crypt_RSA();
//$rsa->setPassword('password');
$rsa->loadKey('...'); // private key
$plaintext = '...';
$rsa->setSignatureMode(CRYPT_RSA_SIGNATURE_PKCS1);
$signature = $rsa->sign($plaintext);
$rsa->loadKey('...'); // public key
echo $rsa->verify($plaintext, $signature) ? 'verified' : 'unverified';
?>