PHP 5.6 support for users authentication

I have a Saas product with legacy authentication based on PHP 5.6 .
I would like to switch the user authentication to Okta on PHP5.6 and later after dropping the old framework to switch to PHP 7.x

The latest Libraries seem to support PHP 7.x.
How can I use Okta with PHP5.6 ? Is there legacy SDK I can use?
Any other alternatives (like using JS only authentication) ?