さすがにそれはロジックがダメすぎまいか...
<?php
$start = microtime(true);
$pn = array(2);
for ($i = 3; $i < 5000; $i +=2) {
foreach ($pn as $n) {
if ($i % $n == 0) {
continue 2;
}
}
$pn[] = $i;
}
$end = microtime(true);
echo implode("\n", $pn), "\n";
printf("%d prime numbers found.\n", count($pn));
printf("%0.6fsec.\n", $end - $start);
?>
microtime(true) は PHP5 限定な。