With PHP 8.5.0 and igbinary 3.2.16 I get the following error message:

In file included from /.../ext/igbinary/src/php7/igbinary.c:52:
/.../ext/igbinary/src/php7/php_igbinary.h:35:10:
fatal error: ext/standard/php_smart_string.h: 
File or directory not found
   35 | #include "ext/standard/php_smart_string.h"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

That error message is valid, because the following files are exists in PHP 8.4.x but not exists in PHP 8.5.x

• ext/standard/php_smart_string.h
• ext/standard/php_smart_string_public.h

There is a PR for this #404
(Maybe I did not find it at the first time, because it is 404 :-) )