The PHPMicro patches needed for this to begin with can be found in my fork
dixyes/phpmicro@master...mpociot:phpmicro:php-84-fixes

Sodium also seems to cause an issue, as the sodium.php file currently tries to remove all lines that contain -Wno-logical-op

Since PHP 8.4, this is no longer a single line though, which produces errors as the -Werror line remains in the config.m4 file after patching:

https://github.com/php/php-src/blob/master/ext/sodium/config.m4#L21-L22

Seems dixyes/phpmicro#3 iconv part is duplicated.

Also, I haven't tested these patches yet, but if they work on Herd, we can use them first.

Yeah that PR is already outdated as it was based on an early alpha release.
I managed to compile 8.4.0RC1 for Herd. I had to disable some extensions that would need further investigation (like duplicate symbols with argon2 and sodium)

And then there are of course a couple of extensions that aren't ready / tagged yet like redis or imagick

If there are extension-independent phpmicro patches that need to be updated, we could use them and test them in advance.

For extensions, continue testing in the draft PR supporting 8.4 (will do) until most extensions are available.