This has been explained a few times. PHP does shallow copies, or
copy-on-write which means that the data is not actually copied until you
change it. That is:
$a = "1234567890";
$b = $a;
internally we do not copy the data from $a to $b until you change $b.
We you use references we have a bit more work to do as we need to decouple
this and indicate that copy-on-write should not be taking place.
Basically things are optimized for the most common case.
-Rasmus
On Tue, 8 Oct 2002, Markas wrote:
> I tried some trivial expirements:
>
> /* here I define an array $big, which I guess would "eat" ~100kb of memory */
> for($i=0; $i<10000; $i++)
> {
> $big[$i] = "1234567890";
> }
>
> /* this func only returns the value it gets as a param...*/
> function f($a){return $a;}
>
> $start = microtime();
>
> /* here all the job is done */
> for ($i=0; $i < 100; $i++){$a = f($big);} /* <--- every iteration I just pass $big
>array to this func and it simply returns it*/
>
> $end = microtime();
>
> /* here I find out the time the job above takes to run, similar to the code from the
>help:*/
> function getmicrotime($time){
> list($usec, $sec) = explode(" ",$time);
> return ((float)$usec + (float)$sec);
> }
>
> $time_start = getmicrotime($start);
> $time_end = getmicrotime($end);
> $time = $time_end - $time_start;
>
> echo "Did nothing in $time seconds";
>
> So the script above takes on my server ~0.00115 sec, so as far as I understand, it
>takes php to copy that $big array which is rather large, at least 100 times... So I
>decided to change the function f($a):
> function f($a){return $a;} changed to function f(&$a){return $a;},
> as you can see, I only wanted to pass that $a param by reference, without copying
>it, so I thought I win in performance and the 100 iterations will work faster, as no
>copying of such a large array $big (which is this time going to be passed by
>refernce) will be involved,... BUT this case the job took ~3.75093 seconds, which is
>3262 times SLOWER !!! I also found out, that while using refernces, the time of job's
>run strictly depends on the $big array dimension, and while NOT using references, the
>time doesn't depend on that, but I thought just on the contrary. I thought, that
>while using references, we dont copy the data and therefore do not depend on that
>data amount, but the example above shows just the opposite results...
>
> What's going on, if anybody gets interested, please explain?
>
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