PHP determines whether a number is prime

Prime number. A natural number greater than 1 that is not divisible by other natural numbers except 1 and itself is called a prime number; otherwise it is called a composite number.

If a number n is divisible by a number between 2 and √n (remainder is 0), then n can be judged to be a prime number. You can start testing from 2 until √n.

In the general field, for a positive integer n, if it cannot be divided by all integers between 2 and , n is a prime number. (Recommended learning: PHP programming from entry to proficiency)

Prime numbers are greater than or equal to 2 and cannot be divided by itself and numbers other than 1

I will not prove it specifically. Give a chestnut:

      16 = 2*8
      16 = 4*4
      16 = 8*2
      √16 = 4

If it is greater than, that is, c=a*b and c=b*a are repeated

function isPrime($n) {
    if ($n <= 3) {
        return $n > 1;
    } else if ($n % 2 === 0 || $n % 3 === 0) { // 排除能被2整除的数(2x)和被3整除的数(3x)
        return false;
    } else { // 排除能被6x+1和6x+5整除的数
        for ($i = 5; $i * $i <= $n; $i += 6) {
            if ($n % $i === 0 || $n % ($i + 2) === 0) {
                return false;
            }
        }
        return true;
    }
}

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