adding php81 compatibility

[elminson]

Fixes

A short description of what this PR does.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

If you have questions, create a GitHub Issue in this repository.

[soundslocke]

See also this similar PR #60 and the update branch: https://github.com/trolley/php-sdk/tree/php-ver-update