Newbie question on reference

I am new in php, I would like to know what exactly the meaning of adding a "&" symbol before a variable or a function. Eg:

$foo = &new Foo();
OR:
function &getBar() {
..
}

Is there any reference I could read on this?

Thanks.

It's in, of all places, the manual (References Explained).

Thx for the reference, I have read the explaination, but I still don't really understand the difference likes:

$bar = new fooclass(); / $bar = & new fooclass();

**As I know, the first script simply create an instance (copy) of the class, but how does the second script mean?

AND

$foo = find_var($bar); / $foo = & find_var($bar);