Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

That is the behavior of arrays as a whole, not of var_export() or serialize().

https://www.php.net/manual/en/language.types.array.php
> The key can either be an int or a string. The value can be of any type.
>
> Additionally the following key casts will occur:
> * Strings containing valid decimal ints, unless the number is preceded by a +
>   sign, will be cast to the int type. E.g. the key "8" will actually be stored
>   under 8. On the other hand "08" will not be cast, as it isn't a valid decimal
>   integer.