php databse store data error :(

errors

: Undefined array key "gender" in
C:\laragon\www\php phonebook project\crud\School Management System\forms\student\create.php
on line
17


Deprecated
: mysqli_real_escape_string(): Passing null to parameter #2 ($string) of type string is deprecated in
C:\laragon\www\php phonebook project\crud\School Management System\forms\student\create.php
on line

my code

if($_SERVER['REQUEST_METHOD'] == 'POST')
    {
        $firstName = mysqli_real_escape_string($conn, $_POST['first_name']);
        $lastName = mysqli_real_escape_string($conn, $_POST['last_name']);
        $email = mysqli_real_escape_string($conn, $_POST['email']);
        $dob = mysqli_real_escape_string($conn, $_POST['dob']);
        $fatherName = mysqli_real_escape_string($conn, $_POST['father_name']);
        $mobile_no = mysqli_real_escape_string($conn, $_POST['mobile_no']);
        $city = mysqli_real_escape_string($conn, $_POST['city']);
        $district = mysqli_real_escape_string($conn, $_POST['district']);
        $gender = mysqli_real_escape_string($conn, $_POST['gender']);
        $state = mysqli_real_escape_string($conn, $_POST['state']);
        $nationality = mysqli_real_escape_string($conn, $_POST['nationality']);
        $image = __FILE__['image']['name'];
        // $imageType = __FILE__['image']['type'];
        // $imageSize = __FILE__['image']['size'];
        // $imageTemp = __FILE__['image']['temp_name'];
        $errors = [];

        if(strlen($_POST['first_name']) == 0){
            $errors['first_name'] = "First name is required.";
        } 
        
        else if(strlen($_POST['first_name']) <= 3){
            $errors['first_name'] = "First name cann't be less than 3 characters.";
        }
        // //image Validation
        // if($imageType != 'image/jpg' || 'image/jpg')
        // {
        //     $errors['image'] = "Please use image type of jpg/png.";
        // } else if($imageSize > 2000) {
        //     $errors['image'] = "Image should not be more than 2MB.";
        // } else {
        //     move_uploaded_file($imageTemp, __DIR__ . '/../../images/students');
        // }

        if(empty($errors))
        {
            $sql = "INSERT into `students`
            (first_name, last_name, email, dob, father_name, mobile_no, city, district, gender, state, nationality, image)
            values('$firstName', '$lastName', '$email', '$dob', '$fatherName', '$mobile_no', '$city',
                '$district', '$gender','$state', '$nationality', '$image'
            )";
            $query = mysqli_query($conn, $sql);

            $_SESSION['msg'] = "Stored Successfully!";
            // header("location:/");
        }
    }