为什么我收到未声明“ firstSci”的错误

Question

我遇到以下错误： 未定义的索引：第147行的E：\ XAMPP \ htdocs \ School \ SteffenFinal2 \ index1.php中的firstSci

if($selectedGrad == 1 && $selectedSub == 2){
    if (!isset($_COOKIE["firstSci"])) {
    $fS = 0;
    setcookie("firstSci", $fS);
    };

    if($selectedGrad == 1 && $selectedSub == 2 && $_COOKIE["firstSci"] >= 1){
        echo "ERROR SUBJECT HAS ALREADY BEEN SELECTED FOR THIS GRADE PLEASE SELECT ANOTHER";
        return;

        } else {

            $fS = $_COOKIE["firstSci"] + 1;
            setcookie("firstSci", $fS);
            echo $_COOKIE["firstSci"];
            };
    };

};