PHP 大数相加

<?PHP

$a = '234567890';
$b = '111111111111101';
$m = strlen($a);
$n = strlen($b);
$num = $m>$n?$m:$n;
$result = '';
$flag = 0;
while($num--){
$t1 = 0;
$t2 = 0;
if($m>0){
$t1 = $a[--$m];
}
if($n>0){
$t2 = $b[--$n];
}
$t = $t1+$t2+$flag;
$flag = $t/10;
$result = ($t%10).$result;
}
echo $result;
echo "\r\n";
echo $a+$b;

?>