How we check whether a number is Prime or not?

In this example we will learn about prime numbers and also how to check whether a number is prime or not. What is the prime number? Prime Number is a special number which is greater than 1 and has only two factors. These numbers can only be divided by 1 and themselves. For example : 3,5,7,11,13...etc..

How to check whether a number is prime or not? Program for checking a number is prime or not.

$number =4; // assign a number to a variable. like 3, 5,7,10
$flag = 0;     //assign a variable into 0

    for ($i = 2; $i <= $number / 2; ++$i) {
        // condition for non-prime
        if ($number % $i == 0) {
            $flag = 1;
            break;
        }
    }
    if ($number == 1) {
        print_r("1 is neither prime nor composite.");
    }
    else {
        if ($flag == 0)
            print_r("$number is a prime number.");
        else
            print_r("$number is not a prime number.");
    }
?>