Hey,
> In ed355dec (PATCH 2/3) the following line is what makes opcache actually work:

What is ed355dec ? I don't have any commit with that reference in
Buildroot. If it's the reference of the commit of PATCH 2/3 in your
Buildroot Git repository, then it doesn't make sense to reference it,
as this commit won't have the same hash when we'll apply your patch,
i.e ed355dec means nothing expect just to you locally.
I know, which is why I referenced both. So I'm referencing the second of three patches in my recently submitted series regarding package/php.
> PHP_CHECK_LIBRARY(rt, shm_unlink, [PHP_ADD_LIBRARY(rt,1,OPCACHE_SHARED_LIBADD)])

And what is this doing ? Again shm_unlink() is no longer in librt, it's
in libc since glibc 2.17 (if I remember correctly).
PHP_CHECK_LIBRARY(rt, shm_unlink, [PHP_ADD_LIBRARY(rt,1,OPCACHE_SHARED_LIBADD)]) is executed for non-cross-compiles, and with my patch also executes for cross-compiles.

Though I haven't looked deeply into the PHP specific autotools macros...
https://github.com/php/php-src/blob/PHP-8.0.9/build/php.m4#L1507

Regards,
Pascal de Bruijn