CDRIVER-5702 deprecated the bson-atomics.h API in libmongoc 1.29. PHPC relied on this for incrementing phongo_num_threads from the GINIT and GSHUTDOWN handlers, in order to determine when it was appropriate to call mongoc_cleanup() and bson_mem_restore_vtable().

We'll need to come up with a suitable replacement.

Note that PHP 8.4 amended the zend_atomic API to support integer operations (php/php-src@1ab4520); however, that's likely too new to rely on.