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

For the record, and excluding the time that it takes:

First:
";
$x = str_repeat('x', 80000);
echo memory_get_usage() . "
\n";      
echo memory_get_peak_usage() . "
\n"; 
echo "
Unset:
";
unset($x);
$x = str_repeat('x', 80000);
echo memory_get_usage() . "
\n";      
echo memory_get_peak_usage() . "
\n"; 
echo "
Null:
";
$x=null;
$x = str_repeat('x', 80000);
echo memory_get_usage() . "
\n";      
echo memory_get_peak_usage() . "
\n";

echo "
function:
";
function test() {
    $x = str_repeat('x', 80000);
}
echo memory_get_usage() . "
\n";      
echo memory_get_peak_usage() . "
\n"; 

echo "
Reasign:
";
$x = str_repeat('x', 80000);
echo memory_get_usage() . "
\n";      
echo memory_get_peak_usage() . "
\n"; 

It returns

First:
438296
438352
Unset:
438296
438352
Null:
438296
438352
function:
438296
438352
Reasign:
438296
520216 <-- double usage.

Conclusion, both null and unset free memory as expected (not only at the end of the execution). Also, reassigning a variable holds the value twice at some point (520216 versus 438352)