Bug report

PHPStan inferres DateInterval::format('%a') to be lowercase-string&non-falsy-string. In case of a difference of the same date the return value can be '0' which is a falsy string.

Code snippet that reproduces the problem

https://phpstan.org/r/1bb0f53b-a6fe-4b17-b6a3-53f0e43be198

Expected output

Based on https://www.php.net/manual/en/dateinterval.format.php

The output might be a numeric string or (unknown).

The expected output: Dumped type: numeric-string|"(unknown)"

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