パフォーマンスのためにPHPのfor文でcountを使うべきではない

<?php
for ( $i = 0; $i < count($array); ++$i){
	doit($array[$i]);
}
?>

for (式1; 式2; 式3) {
	文
}

• 式1の評価は一度だけ、です。
• 式2、式3の評価は、ループするだけ評価(実行)されます。

$i < count($array)

10回ループすれば、count($array)が10回実行されます。PHPは、何も考えず、10回実行してくれます。

さて、どうするのがいいのか？というと、for文の式にcount()を書かずに、for文の前でcount()を計算してあげます。
つまり、こうなります。

<?php
$count=count($array);
for ( $i = 0; $i < $count; ++$i){
	doit($array[$i]);
}
?>

<?php

include ('Benchmark/Timer.php');

function generate_random_array ($max) {
	$array = array ();
	while ($max-- > 0) {
		$array[] = rand (1, 655535);
	}
	return ($array);
}

function for_without_count ( $array ) {
	$count = count ($array);
	$x = 0;
	for ($i = 0; $i < $count; ++$i) {
		$x += $array[$i];
	}
}

function for_with_count ( $array ) {
	for ($i = 0; $i < count($array); ++$i) {
		$x += $array[$i];
	}
}

$num = 1000;

$array = generate_random_array ( $num );

$oTimer =& new Benchmark_Timer();
$oTimer->start ();
for_with_count ($array);
$oTimer->setMarker( 'with' );
for_without_count ($array);
$oTimer->setMarker( 'without' );
$oTimer->stop ();
$oTimer->display ();

?>

------------------------------------------------------
marker    time index            ex time         perct
------------------------------------------------------
Start     1201714387.27995700   -                0.00%
------------------------------------------------------
with      1201714387.28164600   0.001689        71.45%
------------------------------------------------------
without   1201714387.28229200   0.000646        27.33%
------------------------------------------------------
Stop      1201714387.28232100   0.000029         1.23%
------------------------------------------------------
total     -                     0.002364       100.00%
------------------------------------------------------

------------------------------------------------------
marker    time index            ex time         perct
------------------------------------------------------
Start     1201714428.51908800   -                0.00%
------------------------------------------------------
with      1201714428.52260800   0.003520        69.13%
------------------------------------------------------
without   1201714428.52411000   0.001502        29.50%
------------------------------------------------------
Stop      1201714428.52418000   0.000070         1.37%
------------------------------------------------------
total     -                     0.005092       100.00%
------------------------------------------------------

------------------------------------------------------
marker    time index            ex time         perct
------------------------------------------------------
Start     1201714444.30703500   -                0.00%
------------------------------------------------------
with      1201714444.31607100   0.009036        62.85%
------------------------------------------------------
without   1201714444.32128500   0.005214        36.27%
------------------------------------------------------
Stop      1201714444.32141200   0.000127         0.88%
------------------------------------------------------
total     -                     0.014377       100.00%
------------------------------------------------------

------------------------------------------------------
marker    time index            ex time         perct
------------------------------------------------------
Start     1201714471.39839600   -                0.00%
------------------------------------------------------
with      1201714471.41632200   0.017926        63.66%
------------------------------------------------------
without   1201714471.42642200   0.010100        35.87%
------------------------------------------------------
Stop      1201714471.42655400   0.000132         0.47%
------------------------------------------------------
total     -                     0.028158       100.00%
------------------------------------------------------

<?php
$array = array (
		1,
		2,
		3,
		4
	       );
for($i = 0; $i < count($array); ++$i) {
	echo $i, PHP_EOL;
	array_pop($array);
}
?>