sum of great numbers

sum.php

<?php

$a = '2334684234234234234234234234234';
$b =     '593444444444444444444444444';
$tab = strlen($a) > strlen($b)
    ? strlen($a)
    : strlen($b);

printf("\n%' {$tab}s\n", $a);
echo("+\n");
printf("%' {$tab}s\n", $b);
printf("%'_{$tab}s\n", '_');
printf("%' {$tab}s\n", sum($a, $b));

//-----------------------------------------
$a = '1';
$b = '2';

printf("\n%' 30s\n", $a);
echo("+\n");
printf("%' 30s\n", $b);
printf("%'_30s\n", '_');
printf("%' 30s\n", sum($a, $b));


//=========================================

function sum(string $a, string $b)
{
    $posA = strlen($a) - 1;
    $posB = strlen($b) - 1;

    $r = array();
    $posR = 0;
    $rest = 0;

    while ($posA >= 0 and $posB >= 0) {

        $r[$posR] = (string)($a[$posA] + $b[$posB] + $rest);

        processRest($r[$posR], $rest);

        $posA--;
        $posB--;
        $posR++;
    }

    //--we have a first part of number at rest, if one number longer
    if ($posA > $posB) {

        $restNumber = $a;
        $restPos = $posA;
    } elseif ($posA < $posB) {

        $restNumber = $b;
        $restPos = $posB;
    } else {

        $restNumber = '';
        $restPos = -1;
    }
    while ($restPos >= 0) {

        $r[$posR] = (string)($restNumber[$restPos] + $rest);

        processRest($r[$posR], $rest);

        $restPos--;
        $posR++;
    }

    //--revert result of sum and not forget rest
    return $rest
        ? ($rest . strrev(implode($r)))
        : strrev(implode($r));

}

//--calculate and save rest to the next step
function processRest(&$sum, &$rest)
{
    $rest = 0;
    if ($sum > 9) {
        $rest = $sum[0];
        $sum = $sum[1];
    }
}