How to write whether a number is prime in php

Prime numbers: Prime numbers are also called prime numbers. A natural number greater than 1 that is not divisible by other natural numbers except 1 and itself is called a prime number.

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Determine whether a number is prime:

/**
 * 判断一个数是否是质数
 * @param int   $num 所要判断的数
 */
function is_prime($num)
{
    for ($i = 2; $i < $num; $i++) {
        if ($num % $i == 0) {
            return $num . "不是质数";
        }
    }

    return $num . "是质数";
}

echo is_prime(7)."<br>";
echo is_prime(9)."<br>";
echo is_prime(10)."<br>";

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