传递值或引用，递归地排序对象数组

I'm somewhat of a PHP newbie, only doing this as a hobby for the last couple of years.

I have an object of this prototype:

menu_node ->name
          ->order
          ->children

where children may be an array of these same objects - so could go many levels deep. I wrote a simple recursive function that goes through the nesting and executes usort on each ->children:

private function sort_node( $node ) {
    if ( isset($node->children) ) {
        usort(  $node->children,
                function($a, $b) {return ($a->order == $b->order) ? 0 : ( ($a->order < $b->order) ? -1 : 1 );}
        );
        foreach ( $node->children as $child_node ) {
            $this->sort_node( $child_node );
        }
    }
    return $node;
}   

It seems to work fine. But I'm not sure "why" it works fine, which I don't like - I want to know for sure.

I'm worried about what goes on with the passed-in object inside of that foreach loop - am I consistently modifying the memory space of the passed-in object or am I just lucky that my version and configuration of PHP happens to work ok, but it's really indeterminate as to how it would work across other installations?

I guess I am not sure when to use the argument format &$node instead of $node? Same for the iterator variable $child_node, in that loop.

Any insight or confirmation that what I have here is ok, would be appreciated.