$limit = 999999999;
$step = 1000000;
$totalSum = '0';
for ($i = 1; $i <= $limit; $i += $step) {
    $end = min($i + $step - 1, $limit);
    $numberString = '';
    
    for ($j = $i; $j <= $end; $j++) {
        $numberString .= $j;
    }
    
    $totalSum = bcadd($totalSum, $numberString);
}
echo $totalSum;

$limit = 999999999;
$numberString = '';
for ($i = 1; $i <= $limit; $i++) {
    $numberString .= $i;
}
echo $numberString;