...which only happens if the string is empty.

Or a 0, or false, or null.

Or, if it gets an array (but only an array), it emits a warning and returns NULL.

There are plenty of ways bad data can be passed to a function, return types should be consistent and documented.

http://php.net/manual/en/functions.internal.php
> Note: If the parameters given to a function are not what it expects, such as passing an array where a string is
> expected, the return value of the function is undefined. In this case it will likely return NULL but this is just a
> convention, and cannot be relied upon.

Then why bother specifying return value(s) upon failure on other functions?

Because accidentally trying to soundex an empty string (or something convertable to a string) is understandable but calling it with an array doesn't make sense and suggests possible developer error?

See also <https://github.com/php/php-src/commit/aba0ee71b224ab11a4b940f06dd1146eddbd41a3>.