It's PHP 5.3's namespacing functionality, in a nutshell.
Symfony2 and Lithium are two frameworks I can currently think of that use them extensively. On the one hand, Symfony2 has a quite extreme implementation, and on the other, Lithium's is much more concise. The backslashes can be used to represent a hierarchy.
Namespaces, as others have mentioned. The first question I had when I first encountered that as the Namespace operator was: "Why not the double colon operator?"
Double colon is already being used for static member referencing, and in a few talks I've heard from Rasmus, they weren't able to find a way to overload it for Namespaces.
Another option that was discussed was a triple colon operator, which most were opposed to b/c of it's length. I personally think the backslash is ugly, and I kind of like the triple colon operator better b/c of it's similarity to the double colon operator. Regardless of which one is prettier, I wish the double colon operator could have been overloaded.
IIRC, the reasoning against double colons was that if you had a class and a namespace with the same name, it's impossible to tell whether SomeThing::Do() is calling a function in the SomeThing namespace, or the static Do function on the SomeThing class. One could argue that they could simple not allow namespaces and classes to share names, but I guess they didn't like that idea, either.
I hope I'm stating this right, I must admit it's from a fuzzy recollection.
If you're talking about the German layout and AltGr, then so are the []{} keys, which are already used quite a lot in PHP programming.
It's the main reason why, even though my phyiscal keys are in German layout, I still have the USA layout in software. It's just nice to be able to make [] with a single key press :)
Symfony2 and Lithium are two frameworks I can currently think of that use them extensively. On the one hand, Symfony2 has a quite extreme implementation, and on the other, Lithium's is much more concise. The backslashes can be used to represent a hierarchy.