ext/standard: drop old smart_string headers

[DanielEScherzer]

Functionality was removed in PHP 7.2

[Girgias]

MSTM

[DanielEScherzer]

CC @remicollet - if the builds for various (non-bundled) extensions for 8.5.0alpha3 fail because of an unknown header this might be why, solution is to update to the Zend/ header

[wyattoday]

Main branch of igbinary fails with this commit. https://github.com/igbinary/igbinary

[DanielEScherzer]

Okay, I don't see any GitHub actions CI logs to help identify what files are failing to compile, but since https://github.com/igbinary/igbinary/blob/103c70222f1c0d3e139a74f1163007736d2a8a57/package.xml#L231-L235 indicates that it requires/supports PHP 7.0.0+, you'll need to put some condition on the inclusion to include the right header file based on the version

[wyattoday]

I'm not the owner of the igbinary project, but I'll make a PR tomorrow with the fix (if no one else gets to it first).

I was more just making a note of it for anyone wanting to use igbinary with PHP 8.5.0 alpha3 or newer and wondering why it doesn't compile.