07. Sum Prime Non Prime

<?php
$primeSum = 0;
$nonPrimeSum = 0;

while (1){
    $input = readline();
    if ($input == 'stop'){
        break;
    }
    if ($input < 0) {
        echo 'Number is negative.'. PHP_EOL;
        continue;
    }
    if ($input == 1) {
        $nonPrimeSum++;
        continue;
    }
     $isPrime = true;
    for($i = 2; $i < $input; $i++){
        if($input % $i == 0){
            $isPrime = false;
            break;
        }
    }

    if ($isPrime){
        $primeSum+= $input;
    }else {
        $nonPrimeSum += $input;
    }
}

echo "Sum of all prime numbers is: $primeSum\n";
echo "Sum of all non prime numbers is: $nonPrimeSum\n";