How to fix an error in a php variable? Server gives 500 error

I have a code that is responsible for add money to account. There are tariff plans on the page of the site, when you click pay, the user switches to paying the bill. The code is as follows:

   public function pay(Request $request)
    {
        $ticketID = $request->get('tickID');
        if($ticketID = 1){
            $sum = 32;
        }elseif($ticketID = 2){
            $sum = 64;
        }elseif($ticketID = 3){
            $sum = 320;
        }elseif($ticketID = 4){
            $sum = 640;
        }elseif($ticketID = 5){
            $sum = 1600;
        }elseif($ticketID = 6){
            $sum = 3840;
        }elseif($ticketID = 7){
            $sum = 6400;
        }elseif($ticketID = 8){
            $sum = 9600;
        }
        $u = $this->user->steamid64;
        $merchant_id = '423';
        $secret_word1 = '432';
        $sign = md5($merchant_id.':'.$sum.':'.$secret_word1.':'.$u);
        $url = 'https://www.free-kassa.ru/merchant/cash.php?m='.$merchant_id.'&oa='.$sum.'&o='.$u.'&s='.$sign.'&lang=ru&i=&em=';
        $returnValue = [
            'redirect' => $url
        ];
        return $returnValue;
    }

    function getIP() {

        if (isset($_SERVER['HTTP_X_REAL_IP'])) return $_SERVER['HTTP_X_REAL_IP'];

        return $_SERVER['REMOTE_ADDR'];

    }

    public function payaccept(Request $request)
    {
        $merchant_id = '534';
        $secret_word2 = '423423';


        if (!in_array(getIP(), array('144.76.93.115', '144.76.93.119', '78.47.60.198'))) die('hacking attempt!');

        $sign = md5($merchant_id.':'.$_REQUEST['AMOUNT'].':'.$merchant_secret_2.':'.$_REQUEST['MERCHANT_ORDER_ID']);

        if ($sign != $_REQUEST['SIGN']) die('wrong sign');
        $summ = $_REQUEST['AMOUNT'];
        $user = User::Where('steamid64',$_REQUEST['MERCHANT_ORDER_ID']);
        if ($summ = 32) {
            $user->money += $summ;
        }elseif ($summ = 64) {
            $user->money += $summ;
        }elseif ($summ = 320) {
            $user->money += $summ;
        }elseif ($summ = 640) {
            $user->money += round($summ*1.03);
        }elseif ($summ = 1600) {
            $user->money += round($summ*1.05);
        }elseif ($summ = 3840) {
            $user->money += round($summ*1.1);
        }elseif ($summ = 6400) {
            $user->money += round($summ*1.15);
        }elseif ($summ = 9600) {
            $user->money += round($summ*2);
        }
        $user->save();


        die('YES');
    }

Function pay forms a link, amount and digital signature. Function payaccept the function processes the payment and must enter payment information into the database.

The problem is in line $user = User::Where('steamid64',$_REQUEST['MERCHANT_ORDER_ID']); i know it with help var_dump().

This line produces a variable steamid64, but can't fint in a DB. How i can fix it?