$sayi1 = '8928939849823';
$sayi2 = '8928939849823';

$carp = $sayi1 * $sayi2;

$return = number_format($carp, 0);
$return = str_replace(",", "", $return);

echo $carp . '<br />';
echo $return;

// 7.9725966841757E+25
// 79725966841757180763308032