primes.php

<?php



if ($argc != 2) {
    print "Usage:: primes.php <max>" . PHP_EOL;
    exit (1);
}

$i = 1;
$max = intval($argv[1]);
echo "Finding primnumber below " . $max . PHP_EOL;
for ($i =1; $i < $max; $i++) {
    if (isPrime($i)) {
        printf("%d is a prime!\n", $i);
    }
}


function isPrime($num) {
    //1 is not prime. See: http://en.wikipedia.org/wiki/Prime_number#Primality_of_one
    if($num == 1)
        return false;

    //2 is prime (the only even number that is prime)
    if($num == 2)
        return true;

    /**
     * if the number is divisible by two, then it's not prime and it's no longer
     * needed to check other even numbers
     */
    if($num % 2 == 0) {
        return false;
    }

    /**
     * Checks the odd numbers. If any of them is a factor, then it returns false.
     * The sqrt can be an aproximation, hence just for the sake of
     * security, one rounds it to the next highest integer value.
     */
    for($i = 3; $i <= ceil(sqrt($num)); $i = $i + 2) {
        if($num % $i == 0)
            return false;
    }

    return true;
}