See: https://twitter.com/Stof70/status/575694424300122112

Ensuring extension names are consistent will make it easier for packages, including our own PHPLIB, to depend on "ext-mongodb", or whatever we go with. We probably don't want to re-use "mongo", since that conflicts with the 1.0 driver.

I believe Composer does allow dependence on extension versions, so we could use "ext-mongo" in conjunction with a version number, but that would require HHVM starting at a 2.0 release. Typically, libraries do not depend on extension versions, though.

One concern I had with this was allowing a mongofill library to still operate. Apparently, that is already supported, as Composer allows a library to "provide" an extension from a dependency resolution standpoint.