WordPress 5.9 and PHP 8.0-8.1

PHPPHP The web scripting language in which WordPress is primarily architected. WordPress requires PHP 5.6.20 or higher 8.1 was released on 25 November 2021. PHP 8.1 contains new features, performance improvements, deprecations, and backward compatibility breaks. For more information, see the PHP 8.1 release page, changelog, and migration guide.

WordPress is not fully compatible with PHP 8.0 or 8.1. All remaining known PHP 8.1 issues are deprecation notices.

Please note, a deprecation notice is not an error, but rather an indicator of where additional work is needed for compatibility before PHP 9 (i.e. when the notices become fatal errors). With a deprecation notice, the PHP code will continue to work and nothing is broken.

The following is a breakdown of changes in WordPress CoreCore Core is the set of software required to run WordPress. The Core Development Team builds WordPress. that pluginPlugin A plugin is a piece of software containing a group of functions that can be added to a WordPress website. They can extend functionality or add new features to your WordPress websites. WordPress plugins are written in the PHP programming language and integrate seamlessly with WordPress. These can be free in the WordPress.org Plugin Directory https://wordpress.org/plugins/ or can be cost-based plugin from a third-party and theme developers need to be aware of and accommodate in their code.

PHP 8.1 Remaining known deprecation notices

Deprecation notices remain in WordPress 5.9 and work will continue in the 6.0 cycle.

What is required to resolve each?

  • Resolution requires a more structural and all-encompassing solution for input validation (i.e. validating the data type and, in some cases, value passed to a function/method) to be architected and implemented to properly fix the underlying bugbug A bug is an error or unexpected result. Performance improvements, code optimization, and are considered enhancements, not defects. After feature freeze, only bugs are dealt with, with regressions (adverse changes from the previous version) being the highest priority. rather than introducing buggy unexpected behavior by haphazardly silencing the deprecation notice.
  • Each notice needs investigation to determine the impact of such changes.
  • Each change requires thorough testing including full coverage happy and unhappy unit/integration tests.

The remaining known deprecation notices include:

  • Functions in the wp-includes/formatting.php file.
  • parse_url() or wp_parse_url() passed directly to a non-nullable in PHP native function without validating a string type is returned (i.e. both will return null when the requested component doesn’t exist within the given URLURL A specific web address of a website or web page on the Internet, such as a website’s URL www.wordpress.org (see the PHP manual)).
  • wp_xmlrpc_server::mw_newPost(): changing content structure default values from null to an empty string. This change requires a review from XMLRPC domain expert and more testing.
  • Upgrade to Requests 2.0.0: Originally planned for WordPress 5.9, but reverted due to issues with WordPress Core’s updater. Currently planned for 6.0 (TracTrac An open source project by Edgewall Software that serves as a bug tracker and project management tool for WordPress. #54504).

To follow or contribute to this ongoing work, see Trac #53465, #53635, #54183, and #54730.

WordPress Core Changes

PHP 8.0 polyfills introduced

The following PHP 8 polyfills are available in WordPress 5.9:

Each polyfill loads into memory when the site is run on PHP versions less than PHP 8.0.

PHP 8.0 named parameters: match parent to child class method signatures

As noted last year in the PHP 8.0 dev note, WordPress Core is not compatible nor supports named parameters:

Using named parameters when calling WordPress functions and class methods is explicitly not supported and highly discouraged until this audit can be completed, as during the audit, parameter names are subject to change without notice. When this audit has been completed, it will be announced in a future developer note.

In 5.9, WordPress Core’s parent and child class method signatures were changed. The parameter names match in the parent and child methods.

If you extend Core classes and choose to support named parameters, you’ll need to audit and change each method being overloaded to ensure the method signatures match the Core class being extended.

To follow or contribute to this review, see Trac #51553, and #50531.

PHP 8.1 readonly() renamed to wp_readonly()

Though originally planned as a reserved keyword, PHP 8.1 changed readonly keyword to a (limited) contextual keyword with potential future plans to deprecate.

WordPress 5.9 renames the readonly() function to wp_readonly(). For PHP 8.0 or earlier, the original readonly() function is loaded into memory but deprecated.


PHP 8.1 return type enforcement and new #[ReturnTypeWillChange] attribute

PHP 8.1 introduces a new #[ReturnTypeWillChange] attribute to silence the deprecation notice for each PHP native interface method where the overloaded method’s return type is incompatible. WordPress Core adds the attribute to each of its instances.

When overloading a PHP native interface method, you should either have the return type declared (in a covariant compatible manner with the PHP native interface method declaration), or add the #[ReturnTypeWillChange] attribute to silence the deprecation notice.

For example, when implementing the ArrayAccess interface, ensure the required methods are compatible or add the attribute as follows:

 * @param mixed $offset The offset to check for.
 * @return bool True on success or false on failure.
public function offsetExists( $offset ) {
	// the code

 * @param mixed $offset The offset to retrieve.
 * @return mixed The offset’s value.
public function offsetGet( $offset ) {
	// the code

 * @param mixed $offset The offset to assign the value to.
 * @param mixed $value  The value to set.
public function offsetSet( $offset, $value ) {
	// the code

 * @param mixed $offset The offset to unset.
public function offsetUnset( $offset ) {
	// the code

See the PHP RFC: Add return type declarations for internal methods.

PHP 8.1 MySQLi default error mode changed

PHP 8.1 changed the default mysqli error mode from silent to fatal error.

Prior to PHP 8.1, the default error handling mode of MySQLi was silent (i.e. MYSQLI_REPORT_OFF). An error in the extension, database, query, or the database connection returned false and emitted a PHP warning.

PHP 8.1 changed the default error mode to MYSQLI_REPORT_ERROR|MYSQLI_REPORT_STRICT. An error in the extension, database, query, or the database connection throws a fatal error exception.

WordPress Core has its own error reporting and gracefully handles the database errors by inspecting the error codes. In 5.9, Core sets MySQLi error reporting to off to avoid fatal errors due to uncaught exceptions and maintains the current behavior (see Trac #52825).

If you are using wpdb, 5.9 takes care of this for you. However, if you are not using wpdb, you will need to make changes in your theme or plugins. It is recommended to switch to wpdb.


PHP 8.1 deprecation: passing null to non-nullable PHP native functions parameters

A deprecation notice is thrown when passing a null value to a PHP native function’s parameter that is not declared nullable. The value will continue to be coerced, meaning the PHP behavior has not (yet) changed. However, in PHP 9, a TypeError will be thrown.

For example, passing null to strlen() (which expects a string type) will throw the following notice:

Deprecated: strlen(): Passing null to parameter #1 ($string) of type string is deprecated in .. on line ..

The deprecation notice identifies where an underlying bug exists in the code base. Rather than merely silencing the deprecation, the approach taken in WordPress Core is to fix each underlying bug notice through input validation (i.e. validating what is passed to the function) or targeted guarding to skip processing for type mismatches.

As noted previously, deprecation notices remain in WordPress Core. These remaining deprecation notices will require a more structural and all-encompassing architectural solution and tests to avoid haphazardly silencing the deprecation notice while potentially introducing buggy, unexpected behavior.

To follow or contribute to this effort, see Trac #54730.

What do you need to do?

Analyze each deprecation notice in your theme or plugin and craft a solution that resolves the bug.

For example, skip the trim() operation when the value to trimmed is not scalar:

if ( is_scalar( $value ) ) {
	$value = trim( $value );

Check that a string type is returned from the following function before passing the returned value to a PHP native function:

  • parse_url() or wp_parse_url() will return null when the requested component doesn’t exist within the given URL (see the PHP manual).
  • filter_input() will return null if the var_name to get is not set or if using FILTER_NULL_ON_FAILURE flag but the filterFilter Filters are one of the two types of Hooks https://codex.wordpress.org/Plugin_API/Hooks. They provide a way for functions to modify data of other functions. They are the counterpart to Actions. Unlike Actions, filters are meant to work in an isolated manner, and should never have side effects such as affecting global variables and output. fails.

For example:

$url_path     = parse_url( $url, PHP_URL_PATH );
$url_filename = '';
if ( is_string( $url_path ) && '' !== $url_path ) {
	$url_filename = basename( $url_path );


PHP 8.1 deprecation: autovivification on false

Autovivification is the automatic creation of an array. When a variable is defined as a boolean false and then used as an array as shown,

$arr   = false;
$arr[] = 2;

PHP 8.1 throws a deprecation notice:

Deprecated: Automatic conversion of false to array is deprecated in .. on line ..

In PHP 8.1, this code will continue to work; however, in PHP 9.0, it will result in a fatal error.

There are multiple ways to fix your code to not only resolve the deprecation notice but to also avoid a fatal error in the future when PHP 9 is released. Some of ways include:

  • Declare an array before using it, i.e. $arr = array();.
  • Check the returned value to ensure it is an array or at minimum not false before using.
  • When getting an option with get_option(), if an array type is expected, set the default to an empty array.
$options = get_option( ‘some_option’, array() );

For more information, see the PHP RFC: Deprecate autovivification on false.

PHP 8.1 deprecation: implicit incompatible float to int conversion

The implicit conversion of float to int which leads to precision loss will throw a deprecation notice.

$arr = array();
$arr[15.5]; // will throw the deprecation notice because 0.5 is lost


PHP 8.1 other notable deprecations and changes

The following functions are deprecated:

  • Date functions: date_sunrise(), date_sunset(), gmstrftime(), strftime(), strptime().
  • Hash functions:  mhash(), mhash_keygen_s2k(), mhash_count(), mhash_get_block_size(), and mhash_get_hash_name() .
  • Calling key(), current(), next(), prev(), reset(), or end() on objects is deprecated.

The following functions changed:

  • The default of ENT_COMPAT changed to ENT_QUOTES | ENT_SUBSTITUTE in these functions htmlspecialchars(), htmlentities(), htmlspecialchars_decode(), html_entity_decode(), and get_html_translation_table() (Trac #53465).
  • Undocumented operation abbreviations can no longer be passed to version_compare().

See the Migrating from PHP 8.0.x to PHP 8.1.x for a complete list of deprecations and changes.

External Libraries

WordPress 5.9 includes the following updated external libraries:

  • GetID3 1.9.21 which includes preliminary PHP 8.1 support (Trac #54162).
  • SimplePie 1.5.7 which includes significant PHP 8.0 and 8.1 compatibility improvements ( Trac #54659).
  • PHPMailer 6.5.1 which includes preliminary PHP 8.1 support (Trac #53953).

The Requests 2.0.0 library was originally planned for WordPress 5.9. However, due to issues with WordPress Core’s upgrader, it was reverted and planned for 6.0 (Trac #54504).

Test Suites and Tooling

Changes to the WordPress Core PHP Test Suite

What about static tooling? Can it identify incompatibilities?

PHPCompatibility can only find a limited amount of these issues. PHPStan/Psalm/Exakat may find more, but are prone to false positives for non-typed code bases.

A Good Test Suite is the first line of defense

Most of the incompatibilities can be found through a good test suite, which includes full test coverage of both happy and unhappy paths.

Theme and plugin developers are encouraged to extend your test suites.

WordPress Core’s test coverage is currently less than 10%. You are invited and encouraged to contribute tests to grow test coverage in WordPress.

PHPUnit deprecation notices

In PHPUnit < 9.5.10/8.5.21, if a PHP native deprecation notice was encountered, PHPUnit would:

  1. Show a test which causes a deprecation notice to be thrown as “errored”;
  2. Show the first deprecation notice it encountered;
  3. Exit with a non-0 exit code (2), which will fail a CI build.

As of PHPUnit 9.5.10/8.5.21, if a PHP native deprecation notice is encountered, PHPUnit will:

  1. Show a test which causes a PHP deprecation notice to be thrown as “risky”;
  2. Show all deprecation notices it encountered;
  3. Exit with a 0 exit code, which will pass a CI build.

To follow or contribute to this review, see Trac #54183.

Why is this a problem?

These deprecations will become errors in the next PHP major and they will still need to be fixed. As CI builds pass, the deprecations go unnoticed. This means more deprecations remaining in WordPress Core, and would lead to more reports of deprecation notices from end-users.

What’s changing in WordPress Core?

The single site and multisitemultisite Used to describe a WordPress installation with a network of multiple blogs, grouped by sites. This installation type has shared users tables, and creates separate database tables for each blog (wp_posts becomes wp_0_posts). See also network, blog, site PHPUnit configuration files for WordPress Core now include the following attributes:


How does this solve the problem?

This will cause CI builds to fail when a native PHP deprecation notice is encountered, meaning that deprecation notices can not go unnoticed.

Are there any negatives?

Only the first deprecation notice will be shown for a test, and there may be more issues hiding behind a deprecation.

What’s coming in PHP 8.2

A significant change is coming in PHP 8.2 which could impact your themes and plugins. The  RFC to deprecate dynamic properties was approved. See the RFC for more information.

Props to @costdev and @jrf for contributing to this dev notedev note Each important change in WordPress Core is documented in a developers note, (usually called dev note). Good dev notes generally include: a description of the change; the decision that led to this change a description of how developers are supposed to work with that change. Dev notes are published on Make/Core blog during the beta phase of WordPress release cycle. Publishing dev notes is particularly important when plugin/theme authors and WordPress developers need to be aware of those changes.In general, all dev notes are compiled into a Field Guide at the beginning of the release candidate phase., and to @costdev, @antonvlasenko, @javiercasares, and @andraganescu for reviewing.

#5-9, #dev-notes