feat: add mobile-to-web cross-platform tracking tutorial

[lricoy]

WIP WIP WIP!

I still need to de-llm this a bit but I am opening the PR to see how it looks because my local repo is not working

Checklist

• Words are spelled using American English
• PostHog product names are in title case. It's "Product Analytics" not "Product analytics". If talking about a category of product, use sentence case e.g. "There are a lot of product analytics tools, but PostHog's Product Analytics is the best"
• Titles are in sentence case
• Feature names are in sentence case. It's "Click here to create a trend insight" not "... create a Trend Insight" and so on.
• Use relative URLs for internal links
• If I moved a page, I added a redirect in vercel.json

Article checklist

• I've added (at least) 3-5 internal links to this new article
• I've added keywords for this page to the rank tracker in Ahrefs
• I've checked the preview build of the article
• The date on the article is today's date
• I've added this to the relevant "Tutorials and guides" docs page (if applicable)

[vercel]

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Project	Deployment	Review	Updated (UTC)
posthog	Ready	Preview	Jan 23, 2026 8:01pm

[lricoy]

@codex review this please

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid sending distinct_id/session_id in query params

Using query parameters here means the IDs are sent to your web server and can be recorded in access logs, caches, and referrer headers to third-party resources; if distinct_id is a stable identifier (email/user ID), this leaks PII and can create compliance issues. The cross-domain tutorial in this repo uses a URL hash for this reason; consider using a fragment (#distinct_id=...) or a short‑lived token exchange so the IDs stay client‑side.

Useful? React with 👍 / 👎.

[chatgpt-codex-connector]

Use supported mobile SDK API for session ID

The tutorial calls getSessionId()/sessionId() on the iOS/Android SDKs, but the iOS and Android SDK docs in this repo only document getDistinctId()/distinctId() and do not expose a session‑ID accessor. As written, these examples will not compile for users following the current SDK docs, so they can’t obtain the session_id needed for bootstrapping. Either switch to a supported API (or document how to access $session_id in mobile SDKs) before instructing readers to pass it.

Useful? React with 👍 / 👎.