Евгений

function format_big_numbers($number, $delimiter) {
$len = strlen($number);
if ($len > 3){
    if ($len % 3 == 0) {
        $split = str_split($number, 3);
        $number_with_commas = implode("$delimiter", $split);
        return $number_with_commas;
    }
    else if ($len % 3 == 1) {
        $front = substr($number, 0, 1);
        $split = substr($number, 1, $len - 1);
        $split = str_split($split, 3);
        $number_with_commas = implode("$delimiter", $split);
        $number_with_commas = $front . "$delimiter" . $number_with_commas;
        return $number_with_commas;
    }
    else {
        $front = substr($number, 0, 2);
        $split = substr($number, 2, $len - 2);
        $split = str_split($split, 3);
        $number_with_commas = implode("$delimiter", $split);
        $number_with_commas = $front . "$delimiter" . $number_with_commas;
        return $number_with_commas;
    }
}
else {
    return $number;
}
}
$num = '1234567891234567891234567891234'; 
echo format_big_numbers($num, ","); // output is 1,234,567,891,234,567,891,234,567,891,234