[PHP] Explore PHP 8.1 New Features

[mfulb]

The goal is to explore the implications of any new feature changes with PHP 8.1 and determine any needed implementation tasks (ideally with scope and time estimates).

Find and explore areas of potential benefit from leveraging new features enough to get an idea of the scope and then document the effort in the implementation MMF/issue tickets.

Changes are documented here: UPGRADING.

Part of #258.

Acceptance Tasks:

• Investigate PHP 8.1 RC6 and record findings

[mfulb]

Examination of PHP 8.1 RC6 indicates there is none or very minimal impact on the agent from these changes.