Why is passing by reference slower in this code? - php

Question

I've encountered something that seems like a strange performance problem. Running this code:
<?php
function test_ref(&$test)
{
for ($i = 0; $i < 100000; $i++)
{
$foo = "s" . rand(1, 1000);
if (!array_key_exists($foo, $test))
{
$test[$foo] = array();
}
$test[$foo][] = rand(1, 10);
}
}
function test()
{
$test = array();
for ($i = 0; $i < 100000; $i++)
{
$foo = "s" . rand(1, 1000);
if (!array_key_exists($foo, $test))
{
$test[$foo] = array();
}
$test[$foo][] = rand(1, 10);
}
return $test;
}
$scriptstart = microtime(true);
$test = array();
test_ref($test);
$sum = 0;
foreach ($test as $key => $val)
{
foreach ($val as $val2)
{
$sum += $val2;
}
}
echo "sum " . $sum . "<br>";
$scriptelapsed = microtime(true) - $scriptstart;
echo "time taken " . $scriptelapsed . "<br>";
$scriptstart = microtime(true);
$test = test();
$sum = 0;
foreach ($test as $key => $val)
{
foreach ($val as $val2)
{
$sum += $val2;
}
}
echo "sum " . $sum . "<br>";
$scriptelapsed = microtime(true) - $scriptstart;
echo "time taken " . $scriptelapsed . "<br>";
?>
I get these results:
sum 548521
time taken 12.37544798851
sum 551236
time taken 0.29530310630798
What's going on here? It seems to be connected to the fact that I insert sub-arrays into the array, though I don't see why passing by reference should be that much slower.
(this is on PHP Version 5.3.3-7+squeeze14 with Suhosin Patch 0.9.9.1)
(edit: fixed using of unset variables, still the same result)

Answer 1

You're accessing values from another scope - that's always slower than only using ones that are defined within the function itself. Here's a nice blog post explaining it, even though that's not its primary topic: PHP internals: When does foreach copy?

Answer 2

It is just my guess but I think we could explain it like this:
when using no reference a local variable is created (that is directly pointing to a memory) and the loop goes like:
$i = 0; $foo = 499; $test[499] = array(); $test[499][] = 2; "commit" directly to a memory
finaly, the value $test is then returned - reading directly from the memory pointer
when using referenced value, the variable passed is like a pointer to a pointer (that is finally pointing to a memory)
in this case the loop looks like:
$i = 0; $foo = 354; $test[354] = array(); $test[354][] = 7; "commit" to a memory via pointer to a memory pointer
I guess therefore there is at least one more step neccessary when working with referenced variables...