It's deprecated in PHP 8.1, so in a lot of places, one could see notices like urlencode(): Passing null to parameter #1 ($string) of type string is deprecated.
Current unique issues from composer phpunit:unit only:
MediaWiki\Tests\Rest\Handler\UserContributionsHandlerTest::testThatParametersAreHandledCorrectlyForMeEndpoint with data set #0 (array()) urlencode(): Passing null to parameter #1 ($string) of type string is deprecated tests/phpunit/unit/includes/Rest/Handler/HandlerTestTrait.php:62 includes/Rest/Handler.php:103 includes/Rest/Handler/UserContributionsHandler.php:92 includes/Rest/Handler/UserContributionsHandler.php:31 tests/phpunit/unit/includes/Rest/Handler/HandlerTestTrait.php:151 tests/phpunit/unit/includes/Rest/Handler/UserContributionsHandlerTest.php:175 tests/phpunit/MediaWikiUnitTestCase.php:108
SiteConfigurationTest::testSiteFromDb str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated includes/SiteConfiguration.php:515 tests/phpunit/unit/includes/SiteConfigurationTest.php:136 tests/phpunit/MediaWikiUnitTestCase.php:108
FileBackendGroupTest::testGuessMimeInternal with data set "No extension" ('foo', null, null, null, null) strlen(): Passing null to parameter #1 ($string) of type string is deprecated tests/phpunit/unit/includes/filebackend/FileBackendGroupTestTrait.php:413 tests/phpunit/MediaWikiUnitTestCase.php:108
- This test shouldn't be marked as unity at all, it depends on wfExpandUrl() and therefore $wgServer.
HttpRequestFactoryTest::testCreate substr(): Passing null to parameter #1 ($string) of type string is deprecated includes/GlobalFunctions.php:782 includes/GlobalFunctions.php:490 includes/http/MWHttpRequest.php:101 includes/http/GuzzleHttpRequest.php:62 includes/http/HttpRequestFactory.php:120 tests/phpunit/unit/includes/http/HttpRequestFactoryTest.php:91 tests/phpunit/MediaWikiUnitTestCase.php:108