function fineComper($list,$target){
    foreach($list as $index => $item){
        $tmpIndex = $index;
        $nextTarget = $target - $item;

        //find
        $tmpList = $list;
        unset($tmpList[$index]);
        $tmpIndex2 = array_search($nextTarget, $tmpList);
        if ($tmpIndex2 != false) {
            return [$index,$tmpIndex2];
        }
    }
}

$list = [2,7,11,15];
$target = 9;
print_r(fineComper($list,$target));

$list = [3,2,4];
$target = 6;
print_r(fineComper($list,$target));

$list = [3,3];
$target = 6;
print_r(fineComper($list,$target));

<?php
function numToStr($num){
    $tmp = number_format($int);
    $tmp = str_split($tmp);
    $tmp = array_reverse($tmp);
}

function getList($list){
    $transNum = [];
    foreach ($list as $ele){
        $transNum[] = numToStr($ele);
    }
    return $transNum;
}

$input = [123, 1234, -1234, 123456, 12345600, 123405600];
$ans = ["321", "4,321", "-4,321", "654,321", "654,321", "6,504,321"];

$tmp = -123456;
// $tmp = number_format($int);
// var_dump($tmp);
if ($int * -1){
    $status = true;
    $tmp *= -1;
}
$tmp = str_split($tmp);
// var_dump($tmp);
$tmp = array_reverse($tmp);
// var_dump($tmp);

if ($status){

}