How signature file data with RSA 2048 & digest sha256? - php

Question

I am generating a RSA signature in PHP By PHPSECLIB library. According to the instructions, the signature digest must be sha256. Should the digest be set when the key is generated?
I generated Pub/Pri key by blow code: (It should be 2048 and .perm)
$new_key_pair = openssl_pkey_new(array(
"private_key_bits" => 2048,
"private_key_type" => OPENSSL_KEYTYPE_RSA,
));
openssl_pkey_export($new_key_pair, $private_key_pem);
$details = openssl_pkey_get_details($new_key_pair);
$public_key_pem = $details['key'];
file_put_contents('private.pem', $private_key_pem);
file_put_contents('public.pem', $public_key_pem);
I sign data by blow code:
$file_name = "x.json";
$sign_file = "signed/" . $file_name . ".sign";
$data = file_get_contents('files/' . $file_name);
$privatekey = file_get_contents('private.pem');
$rsa = new Crypt_RSA();
$rsa->loadKey($privatekey);
$rsa->setHash('sha256');
$signature = $rsa->sign($data);
file_put_contents($sign_file, $signature);
exit("signed.");
and verify it by
$file_name = "x.json";
$sign_file = "signed/" . $file_name . ".sign";
$publickey = file_get_contents('public.pem');
$data = file_get_contents('files/' . $file_name);
$signature = file_get_contents($sign_file);
$rsa = new Crypt_RSA();
$rsa->setHash('sha256');
$rsa->loadKey($publickey);
$verify = false;
try {
$verify = $rsa->verify($data, $signature);
} catch (Exception $ex) {
$verify = false;
}
echo "verify: " . (($verify) ? "true" : "false") . "<br>";
exit("finished.");
How to set up a digest? Is the signing process correct?