Add PHP SDK reference variant

[fern-api]

Summary

Adds a complete PHP SDK reference variant mirroring the existing Python SDK documentation.

Changes

• 800 PHP reference pages generated from Python templates with proper PHP syntax:
  • All code examples rewritten in PHP (method chaining with ->, array syntax with =>, proper semicolons)
  • Type annotations converted (Optional[T] → ?T, list[T] → array, dict[K,V] → array, Callable → callable)
  • Method names converted from snake_case to camelCase
  • Import statements converted to PHP use statements
  • All internal links updated from /python/ to /php/
• PHP variant registered in sdks.yml with title "PHP", icon brands fa-php, and navigation layout
• 49 guide pages updated — uncommented PHP-specific Tab blocks and table rows
• Core overview page made language-neutral (removed Python-specific code examples and pip install instructions)

Verification

All 8 automated grep checks pass with 0 matches:

1. No Python slugs in PHP pages
2. No /reference/python/ links in PHP pages
3. No python code fences in PHP pages
4. No Python type syntax (Optional[, dict[, list[, Callable[[, tuple[) in PHP pages
5. No PHP content remaining inside JSX comment blocks in guides
6. All uncommented PHP Tab blocks have matching closing tags

Requested by: Fern Support

[github-actions]

❌ Link Check Failed

Preview: https://signalwire-preview-3dcd5907-5ea6-4fcc-81a7-ecabaac19591.docs.buildwithfern.com/docs

❌ 5 Broken Link(s)

Broken URL	Status	Found On
https://server.com/support	404	https://signalwire-preview-3dcd5907-5ea6-4fcc-81a7-ecabaac19591.docs.buildwithfern.com/docs/sdks/guides/mapping-numbers
https://server.com/sales	404	https://signalwire-preview-3dcd5907-5ea6-4fcc-81a7-ecabaac19591.docs.buildwithfern.com/docs/sdks/guides/mapping-numbers
https://server.com/billing	404	https://signalwire-preview-3dcd5907-5ea6-4fcc-81a7-ecabaac19591.docs.buildwithfern.com/docs/sdks/guides/mapping-numbers
https://github.githubassets.com/	404	https://github.com/signalwire/docs/issues/new/choose
https://your-server.com/agent	Network error: Network unreachable. Check internet connection or VPN settings (error sending request for url (https://your-server.com/agent))	https://signalwire-preview-3dcd5907-5ea6-4fcc-81a7-ecabaac19591.docs.buildwithfern.com/docs/sdks/guides/phone-numbers

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