A formula to find prime numbers in a loop

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Here’s a little function that I found: (http://icdif.com/computing/2011/09/15/check-number-prime-number/) Seemed to work for me!

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.
     */
    $ceil = ceil(sqrt($num));
    for($i = 3; $i <= $ceil; $i = $i + 2) {
        if($num % $i == 0)
            return false;
    }

    return true;
}

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