Restoring the BSON memory allocation vtable in _mongoc_init_dtor() (before cleanup is invoked) will ensure that any resources allocated during the ctor are freed appropriately. This will resolve PHPC-632 for those using the PHP driver with a system install of libmongoc.

Per bjori's comment in CDRIVER-1160:

diff --git a/src/mongoc/mongoc-init.c b/src/mongoc/mongoc-init.c
index aee1f6e..c2e421d 100644
--- a/src/mongoc/mongoc-init.c
+++ b/src/mongoc/mongoc-init.c
@@ -171,6 +171,7 @@ static void _mongoc_init_dtor (void) __attribute__((destructor));
 static void
 _mongoc_init_dtor (void)
 {
+   bson_mem_restore_vtable ();
    mongoc_cleanup ();
 }
 #endif

When embedding mongoc phongo should still compile with MONGOC_NO_AUTOMATIC_GLOBALS, but hopefully this patch can make standard mongoc shared build work