oop - What does it mean to start a PHP function with an ampersand?

I'm using the Facebook library with this code in it:

class FacebookRestClient {
...
    public function &users_hasAppPermission($ext_perm, $uid=null) {
        return $this->call_method('facebook.users.hasAppPermission', 

        array('ext_perm' => $ext_perm, 'uid' => $uid));
    }
...
}

What does the & at the beginning of the function definition mean, and how do I go about using a library like this (in a simple example)

Answer

An ampersand before a function name means the function will return a reference to a variable instead of the value.

Returning by reference is useful when
you want to use a function to find to
which variable a reference should be
bound. Do not use return-by-reference
to increase performance. The engine
will automatically optimize this on
its own. Only return references when
you have a valid technical reason to
do so.

See Returning References.