Prime number check.

is_prime.php

<?php

function is_prime(int|float $n): bool {
    if ($n == 2)
        return true;

    if ($n < 2 or $n % 2 == 0)
        return false;

    for ($i = 3; $i <= sqrt($n); $i++) {
        if ($n % $i == 0)
            return false;
    }

    return true;
}

$a = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71];
$b = [];
for ($i = 0; $i < 72; $i++) {
    if (is_prime($i)) {
        $b[] = $i;
        echo "$i ";
    }
}
echo "\n";
echo ($a === $b) ? "true" : "false";

црфе шы <= sqrt($n)
what if n* false?

what is $a is already defined?
sorry for црфе шы