The Unified URI options spec states:

Drivers MUST clearly document the conditions under which TLS is enabled implicitly

Before 1.6.0, the PHP driver never implicitly enabled TLS options (re: driver options); however, this has changed now that we document TLS options supported in the connection string and URI options (as of PHPC 1.6.0 and libmongoc 1.15).

Documenting this would also allow us to clarify the current, inconsistent behavior and our planned change for 1.7.0 (see: PHPC-1446).