在我添加代码来检查数据库中是否已经存在该用户之前，脚本在添加新用户时工作得很好，这是我无法理解为什么它不工作的部分。这是一个只测试android应用程序的环境，所以我更喜欢最简单的版本。

<?php

require_once 'connect_db.php';
$conn = new mysqli($hostname, $admin_name, $admin_pwd, $database);
if($conn->connect_error) {
    die($conn->connect_error);
}

if(isset($_REQUEST["username"]) && isset($_REQUEST["email"]) && isset($_REQUEST["password"]) && isset($_REQUEST["user_type"])) {
    echo "Welcome ". $_REQUEST['username']. "<br />";

    $name = $_REQUEST['username'];
    $email = $_REQUEST['email'];
    $pwd = $_REQUEST['password'];
    $user_type = $_REQUEST['user_type'];

    // first check if user already exists
    $query = "SELECT 1 FROM users WHERE `EMAIL` = '$email'";
    $result = $conn->query($query);
    if (!$result) die("Database access failed: " . $conn->error);

    if (mysqli_num_rows($result) > 0) {
        echo "Error inserting user into database. User already exists";
    } else {
        // then do insert new user
        $query_insert_user = "INSERT INTO users (`NAME`, `EMAIL`, `PWD`, `USER_TYPE`) VALUES ('$name', '$email', '$pwd', '$user_type')";

        if ($conn->query($query_insert_user) === TRUE) {
            echo "New record created successfully";
        } else {
            echo "Error: " . $query_insert_user . "<br>" . $conn->error;
        }
    }

    $result->close();
    $conn->close();

} else {
    echo "Error inserting user into database. Missing user parameters";
}

?>