为什么我的函数总是返回false？

Question

why does my function always return false? i think the problem is caused by the isset function but i really dont know how to fix it

$big = array(
2,3,5,7,11,13,17,19,23
,29,31,37);

$fbig = array_flip ($big);


function isprime($n){
    if($n < 2){
        return FALSE;
    }
    if($n > 2147483647){
        return FALSE;
    }
    if($n < 46341){ 
        if(isset($fbig[$n])){


            return TRUE;
        } else {
            return FALSE;
        }
    }
}

$b = 11;
if(isprime($b)){echo "lol";}

Answer 1

if(isset($fbig[$n])){

This line is the problem.

1. What you want to check is not isset($fbig[$n]) (which checks if there is something in the array at the index $n) but in_array($n, $fbig) (which checks if the array $fbig contains the value $n).

2. The array $fbig is not in the scope of the function since it's defined outside. But you can pass it:

if(isprime($b, $fbig)){echo "lol";}

should work just fine.