You can do that. Of course, PHP's native types are quite limited, but a phpdoc syntax should work with static analysis tools. For instance:

    /** @psalm-type MyobjType = object{mystring: string} */

    /**
     * @param MyobjType $myobj
     */
    function (object $myobj): void

Here are some documentation and examples:

- For Psalm, see https://psalm.dev/docs/annotating_code/type_syntax/utility_t...

- For PHPstan, see https://phpstan.org/writing-php-code/phpdoc-types

It may work in your IDE (autocompletion, etc.) but there is no standard on this side. Some IDE have their own parsers, others use one of the LSPs for PHP.