在循环中查找素数的公式

这是我发现的一个小功能：（http://icdif.com/computing/2011/09/15/check-number-prime-
number/
）似乎对我有用！

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