PHP SDK: pin supported PHP version in CI and package metadata

[countradooku]

Follow-up from the experimental PHP SDK review.

The PHP pre-merge action currently installs distro/default PHP meta-packages (php-cli, php-dev, php-mbstring, php-xml). Those defaults can drift across GitHub runner images, and the repo does not currently state which PHP version the SDK supports.

Proposal

• Install explicit PHP 8.3 packages in .github/actions/php/pre-merge/action.yml:
  • php8.3-cli
  • php8.3-dev
  • php8.3-mbstring
  • php8.3-xml
• Do not use external GitHub Actions for PHP setup; keep setup in-repo / apt-based like the current action.
• Document the supported PHP version in foreign/php/README.md.
• Tighten foreign/php/composer.json with:

"require": {
  "php": ">=8.3"
}

Rationale

Pinning the CI toolchain and Composer platform requirement makes the PHP SDK support policy explicit and avoids accidental changes when runner image defaults change.

Related: #3302
Initial PHP SDK work: #3235

[WaterWhisperer]

Hi, I’d like to work on this issue.

[hubcio]

@WaterWhisperer assigned