Project Euler Problem 3

Project Euler Problem 3: Find the largest prime factor of a composite number.

<?php

function lpf($n)
{
  if ($n < 2) return false;
  for ($p = 2; $p <= 3; $p++) 
    while (!($n%$p))
      if (($n /= $p) == 1) return $p;
  for ($p = 5, $d = 1; pow($p, 2) <= $n; 
    $p += 3-$d, $d *= -1)  
    while (!($n%$p))
      if (($n /= $p) == 1) return $p;
  return $n;
}

echo lpf(600851475143);

?>