Add PHP to the default documentation set (#2706)

[Vam-si-krish]

Summary

• Add PHP to the default documentation slugs exposed by the application.
• Add PHP to Docs.defaults so the backend default set stays in sync.
• Add regression coverage for both default lists.

Closes #2706

Testing

• bundle exec ruby -Itest test/app_test.rb in the project's Ruby 4.0.5 Docker environment — 36 runs, 86 assertions, 0 failures
• Ruby syntax checks for the three touched files
• git diff --check