What's better at freeing memory with PHP: unset() or $var = null

Question

I realise the second one avoids the overhead of a function call (update, is actually a language construct), but it would be interesting to know if one is be

Answer 1

It works in a different way for variables copied by reference:

$a = 5;
$b = &$a;
unset($b); // just say $b should not point to any variable
print $a; // 5

$a = 5;
$b = &$a;
$b = null; // rewrites value of $b (and $a)
print $a; // nothing, because $a = null