Why is calling a function (such as strlen, count etc) on a referenced value so slow?

Question

I\'ve just found something very strange in PHP.

If I pass in a variable to a function by reference, and then call a function on it, it\'s incredibly

Answer 1

I found a bug report from 2005 that describes exactly this issue: http://bugs.php.net/bug.php?id=34540

So the problem seems to be that when passing a referenced value to a function that doesn't accept a reference, PHP needs to copy it.

This can be demonstrated with this test code:

<?php
function CalledFunc(&$aData)
{
    // Do nothing
}

function TestFunc(&$aArray)
{
    $aArray = range(0, 100000);
    $fStartTime = microtime(true);

    for ($iIter = 0; $iIter < 1000; $iIter++)
    {
        CalledFunc($aArray);
    }

    $fTaken = microtime(true) - $fStartTime;

    print "took $fTaken seconds\n";
}

$aArray = array();
TestFunc($sData);
?>

This runs quickly, but if you change function CalledFunc(&$aData) to function CalledFunc($aData) you'll see a similar slow-down to the count example.

This is rather worrying, since I've been coding PHP for quite a while and I had no idea about this issue.

Fortunately there's a simple workaround that is applicable in many cases - use a temporary local variable inside the loop, and copy to the reference variable at the end.

Answer 2

So, taking your answer already given, you can partially avoid this issue by forcing the copy before iterative work (Copying back afterward if the data is changed).

<?php
function TestCountNon($aArray)
{
    $aArray = range(0, 100000);
    $fStartTime = microtime(true);
    for ($iIter = 0; $iIter < 1000; $iIter++)
    {
        $iCount = count($aArray);
    }
    $fTaken = microtime(true) - $fStartTime;

    print "Non took $fTaken seconds\n<br>";
}

function TestCount(&$aArray)
{
    $aArray = range(0, 100000);
    $fStartTime = microtime(true);
    for ($iIter = 0; $iIter < 1000; $iIter++)
    {
        $iCount = count($aArray);
    }
    $fTaken = microtime(true) - $fStartTime;

    print "took $fTaken seconds\n<br>";
}

function TestCountA(&$aArray)
{
    $aArray = range(0, 100000);
    $fStartTime = microtime(true);
    $bArray = $aArray;
    for ($iIter = 0; $iIter < 1000; $iIter++)
    {
        $iCount = count($bArray);
    }
    $aArray = $bArray;
    $fTaken = microtime(true) - $fStartTime;

    print "A took $fTaken seconds\n<br>";
}

$nonArray = array();
TestCountNon($nonArray);

$aArray = array();
TestCount($aArray);

$bArray = array();
TestCountA($bArray);
?>

Results are:

Non took 0.00090217590332031 seconds 
took 17.676940917969 seconds 
A took 0.04144287109375 seconds 

Not quite as good, but a damn lot better.