Per CDRIVER-3562, libmongoc 1.24.0 requires OpenSSL 1.0.1+. This is also mentioned in the docs for Configuring TLS.

PHP 7.4 also requires OpenSSL 1.0.1, but PHP 8.1 bumped to 1.0.2 in php/php-src@c3a6deb.

Since CheckSSL.m4 is used for building bundled libmongoc and libmongocrypt sources, I propose sticking with its dependency. It's unlikely there will be any conflict, as both php.m4 and CheckSSL.m4 use PKG_CHECK_MODULES and should pick up the same OpenSSL library (if available).

Note: the else condition of PKG_CHECK_MODULES falls back to PHP_CHECK_LIBRARY, which does not enforce a library version.