For well-typed codebases or codebases which have stayed up-to-date with the latest PHP versions, there isn’t a big problem [to migrate to PHP 8]. The reality, however, is that WordPress is not such a codebase. – Omar Reiss, CTO yoast.
This quote from the Yoast “WordPress and PHP 8 Compatibility Report” already suggests that you as a WordPress user should not treat the update to PHP 8 lightly. The conclusion of the report reinforces this assumption, as Omar Reiss writes: “By just investigating a subset of breaking changes in PHP 8 we could already confirm this is likely to cause major breakage on sites with unclear origin of that breakage. Oftentimes the error will occur in one place, but is caused by a plugin or theme in a different place, making these issues hard to debug.”
These compatibility issues are also a reason why PHP 8 is not immediately available on all platforms for customers. Especially after major updates, it always makes sense to give the Plugin developers enough time for possible adjustments and to wait for the time being. As soon as there is an assurance that there are no problems with a standard WordPress installation and PHP 8 runs very stable on all systems, PHP 8 would become largely available to all customers.
If you already have access to PHP 8 and want to test the new version with your WordPress website, you should – as before every update – first create a backup.
PHP 8 is a big step forward compared to previous versions. Depending on how you use PHP, you may not immediately see a dramatic improvement in performance. But you should consider updating after testing, once it’s stable, and is available at your WordPress hosting provider. PHP 8 is a natural progression and by implementing the newer version sooner rather than later, you’re creating a solid foundation for future expansion, growth and performance improvements.