I know there is some kind of connection between .NET and UniVerse, by using UniObjects. But I believe that obligates use of .NET, and C# or VB.NET.
Also, I know by reading UniVerse manual that UniVerse can call C/C++ code from within UniVerse, but what I want is the other way around.
Yes, although you have to hunt for it.
On the U2 DevZone site Rocket Software has just launched, there is an article about writing a PDO Driver for PHP access. This driver is written in C (iirc) and has all the source code supplied. It uses the U2 InterCall to interact with the U2 system (UniVerse & UniData). You should be able to use this as a way of making calls to the UniRPC process (this is what UniObjects calls).
You have to sign up to the site to access the article & source code, but sign up is free.