public function big_integer_add($num1,$num2){
$str1 = strval($num1);
$str2 = strval($num2);
$length2 = strlen($str1);
$length3 = strlen($str2);
$length = $length2>$length3?$length2:$length3;
$result = '';
$flag = 0;
while($length--){
$m = 0;
$n = 0;
if($length2>0)
$m = $str1[--$length2];
if($length3>0)
$n = $str2[--$length3];
$tmp = $m+$n+$flag;
$flag = $tmp/10;
$result = ($tmp%10).$result;
}

return doubleval($result);//或者直接返回字符串
}

$a = 9874651230987465123098746512309874651230987465123098746512309874651230987465123098746512309874651230;
$b = 9874651230987465123098746512309874651230987465123098746512309874651230987465123098746512309874651230;
dump($this->big_integer_add($a,$b));
echo "<br>";
dump($a+$b) ;