Skyeng review

<?php

$a = "678234234";
$b = "789";

// выводим для проверки
echo $a + $b . "\n";
	
echo txtSum($a, $b) . "\n";


function txtSum($a, $b)
{
	$extra = 0;
	
	$na = strlen($a) - 1;
	$nb = strlen($b) - 1;
	$nmax = max([$na, $nb]);
	
	$out = "";
	$extra = 0;
	
	for ($i = 0; $i <= $nmax; $i++) {			
		$d1 = $na >= 0 ? $a[$na] : 0;
		$d2 = $nb >= 0 ? $b[$nb] : 0;
		
		$dsum = $d1 + $d2 + $extra;
		$digit = $dsum % 10;
		$extra = ($dsum - $digit) / 10;
		
		$out = $digit . $out;
		
		$na--;
		$nb--;
		
		if ($i == $nmax && $extra > 0)
			$out = $extra . $out;
	}
	
	return $out;
			
}


?>