Let take the number be **600851475143, so to find the largest prime factor run the following code**

**Result :**** 6857**

**<?php**

$max = 600851475143;

$l_num = 0;

for ($j = floor(sqrt($max)) / 2; $j >= 2; $j--)

{

if (!($j % 2)) {

continue;

}

$d = 3;

$x = sqrt($j);

while (($j % $d) && $d < $x) {

$d += 2;

}

if (((!($j % $d) && $j != $d) * 1) == 0)

{

if (!($max % $j))

{

$l_num = $j;

break;

}

}

}

echo $l_num;

?>