Numbers To 100 Words

<?php

$n = intval(readline());

$digits = [ "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"
];
$tens = [
    "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"
];
$rounds = [
    "twenty", "thirty", "fourty", "fifty", "sixty", "seventy", "eighty", "ninety"
];

if ($n < 0 || $n > 100) {
    echo "invalid number";
} else if ($n >= 0 && $n <= 9) {
    echo  $digits[$n];
} else if ($n >= 10 && $n <= 19) {
   echo $tens[$n - 10];
} else if ($n >= 20 && $n <= 99) {
    if ($n % 10 == 0) {
        echo $rounds[$n / 10 - 2];
    } else {
        echo  $rounds[$n / 10 - 2]." ". $digits[$n % 10];
    }
} else {
    echo ("one hundred");
}