It seems like $identifier defined by parameter $identifier on line 30 can also be of type string; however, flipbox\transform\Factory::transform() does only seem to accept null, maybe add an additional type check?
This check looks at variables that have been passed in as parameters and are passed out again
to other methods.
If the outgoing method call has stricter type requirements than the method itself, an issue is raised.
flipbox /
transform
This check looks at variables that have been passed in as parameters and are passed out again to other methods.
If the outgoing method call has stricter type requirements than the method itself, an issue is raised.
An additional type check may prevent trouble.