Problem Three

three.php

$factors = array();

$t = 600851475143;

for ( $d = 3; $d < 8462696833; $d++ ) {
    $calc = $t / $d;
    if ( in_array( $calc, $factors ) ) {
        break;
    }

    if ( is_int( $calc ) ) {
        $factors[] = $d;
        $factors[] = $calc;
    }
}
rsort( $factors );

$prime_factors = array();
$is_prime = true;

foreach ( $factors as $factor ) {
    $is_prime = true;
    for ( $d = 2; $d < $factor; $d++ ) {
        if ( $factor % $d == 0 ) {
            $is_prime = false;
            break;
        }
    }
    if ( $is_prime ) {
        echo $factor;
        break;
    }
}