Adding openssl_sign() stub so PHPStan will correctly deduce $signature variable type which is now shown as mixed instead of string.

Fixes phpstan/phpstan#11791

Author: piotrekkr

Diff for: stubs/openssl.stub

@@ -0,0 +1,13 @@
+<?php
+
+/**
+ * @param-out string $signature
+ */
+function openssl_sign(
+    string $data,
+    string &$signature,
+    #[\SensitiveParameter] \OpenSSLAsymmetricKey|\OpenSSLCertificate|array|string $private_key,
+    string|int $algorithm = OPENSSL_ALGO_SHA1
+): bool
+{
+}