<?php

$str = "abcdefg";
$length = strlen($str);


$start = microtime(true);
for ($m = 1000000; $m > 0; $m--) {
    for ($i = 0; $i < $length; $i++)
    {
        $a = 2*$i;
    }
}
echo "with strlen precalculated: " . (microtime(true) - $start) . "\n";


$start = microtime(true);
for ($m = 1000000; $m > 0; $m--) {
    for ($i = 0; $i < strlen($str); $i++)
    {
        $a = 2*$i;
    }
}
echo "with strlen in the loop: " . (microtime(true) - $start) . "\n";