[wp-hackers] Why pass by reference?

[Micky Hulse]

Hello,

For the second example:

<http://codex.wordpress.org/Function_Reference/add_meta_box#Example>

Why pass $this by reference? Specifically:

array(
    &$this,
    'render_meta_box_content',
),

Occasionally, I'll see WP example code (like above) where the author
passes $this by reference.

Other examples, like:

<http://codex.wordpress.org/Function_Reference/add_action#Using_add_action_with_a_class>

... $this isn't passed by reference.

Sorry if this is a dumb question, but (within the context of coding
OOP for WP) how do I determine if I need to pass $this by reference?
Is there a good WP/PHP rule of thumb?

For the add_meta_box() example, I don't see any obvious reasons why
passing by reference is required and/or better than not. :D

Thanks!
M