it builds fine on aarch64-darwin though:

ma27@darwin01> /nix/store/p466i2srindcxidarr1p7y477qrixc06-php-with-extensions-8.1.28/bin/php -v                            
PHP 8.1.28 (cli) (built: Apr 10 2024 05:53:50) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.1.28, Copyright (c) Zend Technologies
    with Zend OPcache v8.1.28, Copyright (c), by Zend Technologies
ma27@darwin01> uname -a                                                                                                     
Darwin darwin01 23.4.0 Darwin Kernel Version 23.4.0: Fri Mar 15 00:12:41 PDT 2024; root:xnu-10063.101.17~1/RELEASE_ARM64_T8103 arm64

That being said, in case you didn't know: there's a community aarch64-darwin builder that I tend to use for that as well: https://nix-community.org/community-builder/

I think as the most active php maintainer you could easily get access there as well (only if you want to, of course).