A not-null assertion operator is not defined equally in all languages. In some languages, it will throw an Exception if the value is null, hence the "assertion" in the name. Such thing does not exist in PHP as of 8.4, but theres the Null Safe Operator since PHP 8.0 (which works more like the "not-null assertion operator" in TypeScript.

I find this blog post good https://php.watch/versions/8.0/null-safe-operator