Deprecate ext/curl `CURLOPT_DNS_USE_GLOBAL_CACHE`?

Hello Internals, We recently bumped[^1] the minimum required libcurl version supported by the PHP Curl extension to 7.61.0. This aligned with the recent CentOS/RHEL 7, along with other major Linux distros that have already updated to a more recent version of libcurl. I'm writing to the Internals to get some feedback on deprecating a Curl option (a `CURLOPT` constant) that we support in PHP, but is removed on Curl 7.62 and later. Curl supported an option called `CURLOPT_DNS_USE_GLOBAL_CACHE`[^2] that enabled Curl to use a global cache for DNS queries. It was added in Curl 7.9.3 (2002 Jan), marked as obsolete soon after, deprecated in 7.11.1, and removed in Curl 7.62.0[^3] (2018 Oct). The reason is that this functionality was not thread-safe, and was not functional after a few releases. Users can still share DNS caches by reusing Curl handles or by copying the Curl handles. The PHP Curl extension does support this constant, and if PHP is built with thread-safety enabled, setting this option emits a warning with the text "CURLOPT_DNS_USE_GLOBAL_CACHE cannot be activated when thread safety is enabled". I would like to propose to either deprecate this constant, or no-op it. A few points I'd like to draw attention to are that: 1. We have had the no-op'd Curl options before. For example, the `CURLOPT_BINARYTRANSFER` constant deprecated in PHP 8.4[^4] had no effect since PHP 5.1.2. Before PHP 8.4, setting this option had no effect, and it did not trigger any deprecation notices[^5]. 2. GitHub code search[^6] shows that there are close to 1,200 usages of the constant. 3. In all Curl versions since 7.62 and even up until the latest 8.9.0, the `CURLOPT_DNS_USE_GLOBAL_CACHE` exists, but does not do anything[^7]. 4. Applications[^8] and the Curl extension[^9] have special handling to avoid setting this constant in ZTS. If we were to no-op, we could remove these extra handling and clean-up the code base. I'd like to know your opinions; should we deprecate it at the constant-level, or no-op it for now and deprecate in a later version? Thank you, Ayesh. [^1]: https://github.com/php/php-src/pull/13259 [^2]: https://curl.se/libcurl/c/CURLOPT_DNS_USE_GLOBAL_CACHE.html [^3]: https://curl.se/mail/lib-2018-09/0010.html [^4]: https://php.watch/versions/8.4/CURLOPT_BINARYTRANSFER-deprecated [^5]: https://github.com/php/php-src/pull/13114 [^6]: https://github.com/search?q=CURLOPT_DNS_USE_GLOBAL_CACHE+language%3APHP++NOT+path%3A%2F%5Eext%5C%2F%2F+NOT+repo%3AJetBrains%2Fphpstorm-stubs+NOT+repo%3Afacebook%2Fhhvm+NOT+repo%3Athecodingmachine%2Fsafe&type=code [^7]: https://github.com/curl/curl/blob/curl-8_9_0/lib/setopt.c#L254-L256 [^8]: https://github.com/symfony/symfony/blob/v7.1.3/src/Symfony/Component/HttpClient/CurlHttpClient.php#L168-L170 [^9]: https://github.com/php/php-src/blob/php-8.4.0alpha2/ext/curl/interface.c#L1106-L1108