Sum of Digit is Pallindrome or not

gistfile1.php

<?php

function isPalindrome(String $n) 
{ 
    if (strlen($n) == 1) {
        return $n;
    } else {
        $res = $n[0] + isPalindrome(substr($n, 1));
    }

    $res = (string)$res;
    for ($i = 0; $i < strlen($res); $i++) {
        if ($res[$i] != $res[strlen($res)-$i-1]) {
            return 'NO';
        }
    }
    return 'YES';
    
}

$n = 56;
echo isPalindrome($n);