[PHP] Explore PHP 8.1 Build System Changes

[mfulb]

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

Find and explore areas of change enough to get an idea of the scope of any work and then document the effort in the implementation MMF/issue tickets.

Changes are documented here: UPGRADING.INTERNALS.

Part of #258.

Acceptance Tasks:

• Investigate PHP 8.1 RC5 and record findings
• Investigate PHP 8.1 RC6 and record findings

[mfulb]

When upgrading the prototype to use RC4 it was noticed the following warning was issued when the PHP build ran the "configure" script:

configure: WARNING: unrecognized options: --enable-zip, --without-libzip, --enable-maintainer-zts

Upon investigation it was found:

• since libzip is no longer bundled with the PHP sources as of PHP 7.4 the "--enable-zip" and "--without-libzip" options are no longer accepted and the proper way to enable support is now to use "--with-zip".
• the "--enable-maintainer-zts" option has been renamed top "--enable-zts" as of PHP 8.0.

The php build scripts used internally have been updated to reflect these changes.

[mfulb]

Reviewed RC5 - main change is "-Wimplicit-fallthrough=1" is now default for compilers which support the option.
This change affects how PHP itself is compiled and not the agent itself.
The agent uses "-Wextra" which includes "-Wimplicit-fallthrough".

[mfulb]

No new changes with RC6 release.