SDK compatibility with PHP 8.1

[matczar]

🐞 bug report

Behavior:

• What is the issue? (*)
  A customer has reported that the SDK generates warnings when testing on PHP 8.1.

PHP Deprecated: hash_hmac(): Passing null to parameter [#3] ($key) of type string is deprecated in /[...]/lib/includes/sdk/FreemiusWordPress.php on line 260

PHP Deprecated: substr(): Passing null to parameter [#1] ($string) of type string is deprecated in /[...]/lib/includes/fs-core-functions.php on line 549

PHP Warning: Undefined property: stdClass::$plan_id in /[...]/lib/includes/entities/class-fs-site.php on line 122

• What is the expected behavior?
  No warnings

Versions: (*)

• Freemius SDK Version: 2.5.3
• WordPress Version: 6.1
• PHP Version: 8.1

[cliffordp]

PHP 8.1.21
NotificationX | Version 2.7.5 by WPDeveloper
NotificationX Pro | Version 2.8.3 by WPDeveloper

PHP Deprecated:  Automatic conversion of false to array is deprecated in /wp-content/plugins/notificationx-pro/includes/Extensions/Freemius/Freemius.php on line 33