MYSQLI错误无法处理请求

Question

所以我目前遇到一个奇怪的问题。我正在使用我的大学数据库进行注册/登录。但是，由于某种原因，哈希方面似乎完全毁了它。没有它，数据就可以很好地插入到数据库中，但是，一旦尝试进行任何安全性哈希处理，我就会得到“ 500台服务器无法处理此请求”。每当我想要对escape_string进行额外的安全保护时，这也适用。有什么建议吗？

    <?php
if (isset($_POST['register'])) {
    require "loginDB.php";


    $username = ($_POST['userID']);
    $email = ($_POST['userEMAIL']);
    $password = ($_POST['userPWD']);
    $password2 = ($_POST['userPWD2']);
    //  $encrypted = $mysqli->escape_string(password_hash($_POST['userPWD'], PASSWORD_DEFAULT));

    if (empty($username) || empty($email) || empty($password) || empty($password2)) {
        header("Location: ../index.php?error=emptyfields&userID=" . $username . "&email=" . $email);
        exit(); //If user makes a mistake they get sent back and have some of the fields filled in
    } else if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        header("Location: ../index.php?error=invalidemail&userID=" . $username);
        exit();
    } else if ($password != $password2) {
        header("Location: ../index.php?error=passwordcheck&userID=" . $username . "&email=" . $email);
        exit();
    } else {
        $sql = "SELECT username FROM users WHERE username=? ";
        $stmt = mysqli_stmt_init($conn);
        if (!mysqli_stmt_prepare($stmt, $sql)) {
            header("Location: ../index.php?error=sqlerror");
            exit();
        } else {
            mysqli_stmt_bind_param($stmt, "s", $username); //binds the data
            mysqli_stmt_execute($stmt); //runs the data inside the database
            mysqli_stmt_store_result($stmt); //takes result from database
            $check = mysqli_stmt_num_rows($stmt);
            if ($check > 0) {
                header("Location: ../index.php?error=usernametaken&email=" . $email);
                exit();
            } else {
                // $encryptPWD = password_hash($password, PASSWORD_DEFAULT);
                $sql = "INSERT INTO users (username, email, pwd) VALUES ('$username', '$email', '$password')";
                header("Location: ../index.php?registration=successful");
                $stmt = mysqli_stmt_init($conn);
                mysqli_stmt_prepare($stmt, $sql);
                mysqli_stmt_execute($stmt);
                exit();
                //  }
            }
        }
    }
    mysqli_stmt_close($stmt);
    mysqli_close($conn);
} else {
    header("Location: ../index.php");
    exit();
}