refactor: PHP 7.3 compatibility
For PHP 7.3 backward compatibility which resolves #19 . Tested this on PHP7.3 locally and all tests are passing.

@vividvilla @ranjanrak
Thanks for the PR. I Will go through this over weekend and update here.
Thanks for the PR. I Will go through this over weekend and update here.
Any updates?
@pushpak1300 We should also remove the Mixed_ Docblock type enforced in invalidateRefreshToken and modifyOrder functions, when mixed type is already added in docblock comment above these functions.
@ranjanrak any updates?
LGTM. @vividvilla, you can merge this.