It's called a Reference Assignment. It makes the assigned-to variable point to the same value as the assigned-from variable.
In PHP 4, this was fairly common when assigning objects and arrays otherwise you would get a copy of the object or array. This was bad for memory management and also certain types of programming.
In PHP 5, objects and arrays are reference-counted, not copied, so reference assignment is needed much less often. Some programmers still use it 'just in case' PHP for some reason decides a copy makes sense there. But a reference assignment is still valid in other ways, such as with scalar variables, which are normally copied on assignment.
