[1/3] docs: keep the PHP opening tag in API reference samples

[dazzatronus]

Every PHP sample in the API reference is missing its first five lines. Shins runs sanitize-html over the markdown, code blocks included, and only exempts xml blocks, so <?php … > is stripped as if it were an HTML tag. This exempts php blocks too.

tests/reference-code-samples.cjs fails if any PHP sample loses its opening tag, and runs in CI after the docs build.

Verify: pnpm build:docs && node tests/reference-code-samples.cjs → PHP samples: 22 complete.

Stack (merge in order)

	PR	Adds	Why it is separate
1/3 ← this PR	#147	Keeps <?php in rendered PHP samples	A Shins fix, independent of what the samples contain
2/3	#148	A small, valid example for every request body	Spec data that the samples embed; also changes the published bundle
3/3	#149	Current-idiom sample templates and a check that runs them	Depends on the examples; the check and templates must land together

[github-actions]

🎉 This PR is included in version 1.20.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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