php - the difference between unset and = null

Question

From a random php.net post:

If you are doing $whatever = null; then you are rewriting variable's data. You might get memory freed / shrunk faster, but it may steal CPU cycles from the code that truly needs them sooner, resulting in a longer overall execution time.

Apparently this is the undisputed truth so maybe someone would be so kind as to explain.

I mean, what, does unset magically not execute any assembly instructions whereas $whatever = null; does? The answer, as given, is about as useful as saying

$whatever = null resets the buffer and the L1 cache whereas unset clears the buffer and resets the L2 cache.

Techno mumbo jumbo doesn't constitute an answer.

Answer 1

An important difference between both methods is that unset($a) also removes $a from the symbol table; for example:

$a = str_repeat('hello world ', 100);
unset($a);
var_dump($a);

Outputs:

Notice: Undefined variable: a in xxx
NULL

But when $a = null is used:

$a = str_repeat('hello world ', 100);
$a = null;
var_dump($a);

Outputs:

NULL

I ran the code through a benchmark as well and found that $a = null is roughly 6% faster than its unset() counterpart. It seems that updating a symbol table entry is faster than removing it.

Addendum

The other difference (as seen in this small script) seems to be how much memory is restored after each call:

echo memory_get_usage(), PHP_EOL;
$a = str_repeat('hello world ', 100);
echo memory_get_usage(), PHP_EOL;
// EITHER unset($a); OR $a = null;
echo memory_get_usage(), PHP_EOL;

When using unset() all but 64 bytes of memory are given back, whereas $a = null; frees all but 272 bytes of memory. I don't have enough knowledge to know why there's a 208 bytes difference between both methods, but it's a difference nonetheless.