Undocumented change to error behavior in array_walk from PHP 7.4 to PHP 8.0

[RobertGonzalez]

In the documentation for PHP 8.0 backward incompatibility, in the Standard Library section, there is no mention of the change in behavior for array_walk when the first argument passed to it is not of type array|object.

The following code behaves differently between PHP 7.4 and PHP 8.0:

$a = null;
array_walk($a, 'strtolower');

In PHP 7.4 the output is:

Warning: array_walk() expects parameter 1 to be array, null given in <FILENAME> on line <LINE>

In PHP 8.0+ the output is:

Fatal error: Uncaught TypeError: array_walk(): Argument #1 ($array) must be of type array, null given in <FILENAME>: <LINE>

We are currently upgrading our host environments and ran into this issue post-upgrade. Most everything else to look out for is in the documentation, but this one caught us off guard because there is no mention of it in the docs.

[damianwadley]

This isn't just about array_walk: all(ish) built-in functions were changed to throw a TypeError when passed arguments with the wrong types. Which is how userland functions already worked.
https://wiki.php.net/rfc/consistent_type_errors

I'm not seeing it documented in the migration guide though, which is kinda surprising...

[Girgias]

We technically considered passing invalid types UB, and it has a noted somewhere. Probably the reason why we didn't document this in the migration guide.

[Girgias]

As said previously we consider this UB, and it is something that is generally documented in the internal functions documentation