问检查数据库中是否已经存在数据

EN

<?php

$response = array();

if (isset($_POST['user_id'])){

        $id = $_POST['user_id'];
        $fullName = $_POST['user_fullName'];
        $firstName = $_POST['user_firstName'];
        $lastName = $_POST['user_lastName'];
        $birthday = $_POST['user_birthday'];
        $friends = $_POST['user_friends'];
        $totalFriends = $_POST['user_totalFriends'];
        $email = $_POST['user_email'];
        $gender = $_POST['user_gender'];
        $likes = $_POST['user_likes'];
        $events = $_POST['user_events'];
        $hometown = $_POST['user_hometown'];

    require_once __DIR__ . '/db_connect.php';

    $db = new DB_CONNECT();

    $query = mysql_query("SELECT * FROM 'userData' WHERE 'id' = '$id'");
    $user_data = mysql_num_rows($query);

    if(empty($user_data)) {         

        $result = mysql_query("INSERT INTO userData(id, fullName, firstName, lastName, birthday, friends, totalFriends, email, gender, likes, events, hometown) VALUES ('$id', '$fullName', '$firstName', '$lastName', '$birthday', '$friends', '$totalFriends', '$email', '$gender', '$likes', '$events', '$hometown')");

        if($result){
            $response["success"] = 1;
            $response["message"] = "User successfully created";

            echo json_encode($response);
        }

        else{
            $response["success"] = 0;
            $response["message"] = "Error occurred";

            echo json_encode($response);
        }
    }

    else {

        $response["success"] = 1;
        $response["message"] = "User already in database";

        echo json_encode($response);            

    }
}

else{
    $response["success"] = 0;
    $response["message"] = "Required userdata is missing";

    echo json_encode($response);
}