A formula to find prime numbers in a loop
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;
}
You can use this PHP function gmp_nextprime()