<?php
$iterations = pow(10, 5);
$a = range(1, $iterations);

$aStart = microtime(true);
foreach ($a as $ka => $va) {
    $a[$ka] = $va+1;
}
echo 'key: '.number_format(microtime(true) - $aStart, 8);

echo "\n<br>";

$b = range(1, $iterations);

$bStart = microtime(true);
foreach ($b as $kb => &$vb) {
    $vb++;
}
echo 'ref: &nbsp;'.number_format(microtime(true) - $bStart, 8);