2020 WordPress and PHP 8 compatibility report

 On November 26, PHP 8 will be released to the world. PHP 8 is set to become one of the most breaking PHP releases in the history of PHP and will bring along unprecedented challenges for legacy PHP codebases, like WordPress, to fix compatibility. 

Today we bring you a comprehensive report on WordPress and PHP 8 compatibility. In sharing this, we hope to educate and help inform both the WordPress and PHP communities about the state of WordPress and PHP 8. After all, PHP is the technology that powers WordPress and WordPress is by far the largest consumer of PHP.

This report is a team effort by Juliette (a PHP engineer well-respected in both the PHP and WordPress communities), Herre (Chief software architect at Yoast) and myself. 

Table of contents

• Introduction
  • What’s in this report?
  • How come there are so many breaking changes in PHP 8?
  • Isn’t WordPress already compatible with PHP 8?
• Part 1: The most worrisome breaking changes in PHP 8
  • Strict typing on internals in PHP 8
    • Consistent type errors
    • Arithmetic operator type checks
    • Magic methods type checks
    • Numeric string handling
    • Named parameters
    • API changes which could lead to type errors
  • Warnings converted to error exceptions
    • Error level changes unrelated to RFC’s
    • Reclassified engine warnings
    • Fatal error for incompatible method signatures
  • Default error reporting level
  • 7.x deprecations
  • Other breaking changes.
• Part 2: Compatibility challenges
  • Static analysis tools
    • Scanning WordPress with PHPCompatibility
    • Scanning WordPress with Exakat
    • Scanning WordPress with PHPStan
  • Testing
    • Running automated tests on PHP 8
    • Getting the tests to run on PHP 8 for WP Core
    • Code Coverage
  • Testing plugins and themes
    • Integration tests
    • Unit tests
  • Compatibility of external WordPress dependencies
    • External dependencies (maintained)
    • External dependencies (unmaintained)
  • Impact on broader ecosystem
• Part 3: yoast.com case study
  • What did we do?
  • Analysis
    • Warnings coming from anywhere
    • From frequent to rare
    • Things are looking tricky for PHP 8 compatibility on large WordPress sites
• Conclusion

Introduction

What’s in this report?

In the first part of this report we’ll outline the changes in PHP 8 that are likely to significantly impact WordPress and other legacy codebases. In the second part of this report, we’ll try to provide a perspective on past, present and future challenges regarding WordPress and PHP compatibility. At the end of the report we’ve included a case study of Yoast.com, to illustrate what kind of issues are likely to occur for a large WordPress site on PHP 8.