largestprimefactor.php

<?php
$primes = array();
$input = 600851475143;

/* Find the primes */

for($i = 1; $i < 10000; $i++) {
	$prime = gmp_strval(gmp_nextprime($i));

	if(end($primes) != $prime) {
		$primes[] = $prime;
	}
}

/* Find largest factor */

foreach($primes as $prime) {
	if($input % $prime == 0) {
		$output = $prime;
	}
}

echo $output;