The string $needle and optional boolean $ignoreCase parameters of the assertContains() and assertNotContains() functions are deprecated and removed in PHPUnit 9. For assertions that operate on strings, the specific assertStringContainsString(), assertStringContainsStringIgnoringCase(), assertStringNotContainsString(), and assertStringNotContainsStringIgnoringCase() methods should be used.

See Usage of the assertEquals function (phpunit.readthedocs.io) for details.