References Explained
PHP Manual

Unsetting References

When you unset the reference, you just break the binding between variable name and variable content. This does not mean that variable content will be destroyed. For example:

<?php
$a 
1;
$b =& $a;
unset(
$a); 
?>
won't unset $b, just $a.

Again, it might be useful to think about this as analogous to Unix unlink call.


References Explained
PHP Manual