Where to find the documentation of XDebug 2?

TL;DR: Checkout the 2.9.1-release branch of the official xdebug.org repository.

Why bother with XDebug 2?

Yes, in 2025, XDebug 2 is no longer supported. However, there are still situations where using XDebug 2 is legitimate. One of those situations is when you're using an older PHP version that's still supported by the Linux distribution of your choice.

For example, Ubuntu 22.04 LTS is packaging and maintaining PHP 8.1 until the end-of-life of 22.04 — despite the fact that security support for 8.1 officially ends before in January 2026.

Solution

So back to the initial question: Where to find documentation for XDebug 2? (Because the only documentation online is the migration guide from 2 to 3. And that's a rather incomplete view on XDebug 2.)

The solution is to check out the 2.9.1-release branch of the official xdebug.org repository. Once you've done that, you'll find the documentation as HTML pages in html/docs/include/features etc.