检查一个数字是否为素数？

Question

For checking whether a number is a prime or not, wouldn't be enough to check if it is divisible by 2, 3, 5, and 7? While looking at other programs on the internet, I have found that people are checking the factors till the number, or half of the number or till the square root of the number.

if ( ($number%2 ==0) || ($number%3==0) || ($number%5==0) || ($number%7)==0) )
    echo "not a prime";

The above check would suffice right? Any thoughts? ignore the prime numbers 2, 3, 5, 7 for now.

Answer 1

No, it is not enough. For example a prime number 11 is not divisible by 2, 3, 5, and 7. And not a prime number 121 is not divisble by 2, 3, 5, and 7, but is divisible by 11. See the definition of the prime number.