PHP 8 is a major update of PHP and it is common practice to remove deprecations in major versions from the previous range of minor versions. For PHP 8, many of the breaking changes have been deprecated in previous 7.* versions. So for projects that were diligently updated over the years fixing their deprecated API’s, it shouldn’t be hard to upgrade at all.
However, PHP 7.* versions have seen a far larger set of deprecations than previous versions of PHP. Where PHP 5.6 to PHP 7 was a relatively simple migration, going from 7.x to 8 could be very painful, especially for very old codebases, like WordPress and many of the plugins that are available for it. For well-typed codebases or codebases which have stayed up-to-date with the latest PHP versions, there isn’t a big problem. The reality, however, is that WordPress is not such a codebase.
Isn’t WordPress already compatible with PHP 8?
Well… Yes. Sort of. Maybe. We are highly doubtful. It’s really not possible to tell.
WordPress aims to always be compatible with new versions of PHP. Sergey did an amazing job in fixing most of the compatibility issues that could be detected using the available strategies. We’ll definitely dive deeper into what those are and the issues that exist with them. Technically, the compatibility of the current nightly of WordPress with PHP 8 is at the same level as we’re used to from WordPress releases right before a new version of PHP comes out. We believe the testing was as extensive, the fixing was as diligent and the level of fixes was as high as any round of PHP compatibility fixing within WordPress core. See also the call for testing on PHP 8 on WordPress.org.
However, doing what we’ve always done, unfortunately, will not cut it this time. The sheer amount of breaking changes and the type of changes included in PHP 8, plus some added complexities in cross-version tooling, make this compatibility challenge a different beast from what we’ve seen before. This report aims to explain how that is the case.
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