sumStrings.php

<?php
function sumStrings($a, $b) {
    $intMaxLength = strlen((string)PHP_INT_MAX) - 2;
    $aArr = str_split(strrev($a), ($intMaxLength));
    $bArr = str_split(strrev($b), $intMaxLength);
    $add = 0;
    $res = [];
    $aArrLength = (count($aArr));
    foreach ($aArr as $k => $block) {
        $i = (int)(strrev($block)) + (int)strrev($bArr[$k] ?? '0') + $add;
        $add = 0;
        $i = (string)$i;
        if (strlen($i) > strlen($block) && $k !==  $aArrLength - 1) {
            $add = 1;
            $i = substr($i, 1);
        } elseif (strlen($i) < strlen($block)) {
            $i = str_repeat('0', strlen($block) - strlen($i)) . $i;
        }
        $res[] = $i;
    }

    return join(array_reverse($res));
}


$a = '99999';
$b = '99999';

if (strlen($a) > strlen($b)) {
    $c = sumStrings($a, $b);
} else {
    $c = sumStrings($b, $a);
}