How to check which http request to execute first in php backend?

I have received both main response request and backend response request from payment side.

It will run my function to update my transaction table to record payment details. There's will be a unique payment id pass into my function.

for example,

 public function response(Request $request) {
    $id = $request->id_trans;
    $trans = new Transaction;
    $trans->id_trans = $id;
    $trans->....
    $trans->save();
}

public function backend_response(Request $request) {
   $id = $request->id_trans;
   $check_trans = DB::table('transaction')->where('id_trans', $id)->first();
   if($check_trans)
       echo "OK";
       exit;
   else
       // insert here..
}

I found that it by pass my checking condition in backend_response() due to it execute before transaction insert into my table. How can i prevent this issue update my table twice? How can i ensure response() to execute first before backend_response()?