I just took a look at the comment in the documentation on null in php and thought the comment from the link above would be worthy to add to the blog as a memo to myself.
So basically what happens when unset is used to reset a variable’s value is that it destroys the reference and resets only a variable being unsetted.
However, when null is used to reset the variable it reset the value of the variable and keeps relationship. Thus, all reference variables have the same value, which is null.