PHPUnit 9.3.0 introduces two new assertions Assert::assertIsClosedResource() and Assert::assertIsNotClosedResource().

Polyfilling these would be challenging to say the least, as the PHP native functionality used to determine whether a resource is closed or not only became available in PHP 7.2.

Refs:

• assertIsNotResource() and closed resources sebastianbergmann/phpunit#4276
• Add 'resource (closed)' type and assertions for closed resources sebastianbergmann/phpunit#4365
• https://www.php.net/manual/en/function.gettype.php#refsect1-function.gettype-changelog