The closure/anonymous function is missing the native return type declaration. Native return type can help enforce type, which can lead to more readable, reliable code. They ensure that the value is of the specified type at call time, otherwise a TypeError is thrown.
mixedis forbidden PHP-T1006
mixed type already includes
null, so specifying
mixed type as a nullable is redundant and doesn't make any sense. PHP lets you typehint properties, parameters, and return values as
mixed which can contain any value.
To fix this issue, it is recommended to drop
null from the nullable