如何正确编写登录后显示usernname的PHP代码？

我需要帮助写一个代码，显示登录后的用户名！我一直在关注一些视频来帮助我完成这段代码，所以我为任何失误道歉。我是初学者

Profile.php

       <?php 
           require("connect.php");
           require("functions.php");
           if(logged_in())
           {

---

首先，您需要做的是检查用户是否使用所需的详细信息成功登录，这些详细信息主要包含用户名和密码。 之后，您需要在会话中存储用户的用户名。e、 g

---

在需要使用用户名的任何地方，您都可以回显

登录成功后，您将需要创建一个会话。像

<?php
session_start();
$_SESSION['FirstName'] = firstName; //The firstname the user typed in
$_SESSION['LastName'] = lastName; //The lastname the user typed in
?>

您可以将任何信息“保存”到会话中，以便使用

$\u session

变量在其他网页上检索该信息。例如，如果添加

$\u SESSION['userid']=$\u POST['userid']会话后_start（）将保存用户名以备将来使用。使用以下标记在HTML中包含用户名：
或者在注册时向会话添加所需的所有用户数据（缺点：如果用户更新其个人资料，则必须更新数据），或者在每个页面请求中查询登录的用户数据（缺点：再查询一次，从性能角度看，这不重要）。然后，它只是一个if来检查您是否有一个登录的用户，如果有，则回显用户数据（来自会话或从db检索到的数据）Hello JimL！谢谢你，但是我知道你在说什么，我只是不知道如何在我的代码中写出来（如果您已经编写了上面的代码，它应该相当简单。只是稍加修改。如果您考虑将此代码联机，我强烈建议您使用MySQL和使用准备好的参数化查询。然后，可以删除所有那些“逃逸字符串”。好的，再次感谢你，JimL！嗯，我很抱歉。但是每当我去放置$\u会话['userid']=$userid我得到一个未识别的变量。这是很棒的dealwap！非常感谢！实际上我还有一个问题。我应该把这段代码放在哪里？在connect.php中？把它放在处理您登录的文件中。它应该只有在成功登录后才执行。非常感谢，辛普森！谢谢！要显示我需要做的就是echo？实际上我还有一个问题。我将把这个代码放在哪里？在connect.php中？您已经在登录代码中设置了一个会话变量，这将是向会话添加更多数据的合适位置。嗯，对不起。但是每当我将$_session['userid']=$userid放入时，我都会得到一个未识别的变量。您是否在if中添加了会话（isset？啊，非常感谢你，Michael！实际上我还有一个问题。我应该把代码放在哪里？在connect.php中？
<?php 
    function email_exists($email, $con)
    {
    $result = mysqli_query($con,"SELECT id FROM users WHERE email='$email'");

    if(mysqli_num_rows($result) == 1)
    {
        return true;
    }
    else
    {
        return false;
    }

}

function logged_in()

{   

        if(isset($_SESSION['email']) || isset($_COOKIE['email']))
        {
            return true;
        }
        else
        {
            return false;
        }
}

<?php

$con = mysqli_connect("localhost","root","****","database");

if(mysqli_connect_errno())
{
    echo "Error occured while connecting with database ".mysqli_connect_errno();
}
session_start();

include("connect.php");
include("functions.php");

if(logged_in())
{
    header("location:profile.php");
    exit();
}

$error = "";

if(isset($_POST['submit']))
{
    $firstName = mysqli_real_escape_string($con, $_POST['fname']);
    $lastName = mysqli_real_escape_string($con, $_POST['lname']);
    $email = mysqli_real_escape_string($con, $_POST['email']);
    $userid = mysqli_real_escape_string($con, $_POST['userid']);
    $password = $_POST['password'];
    $passwordConfirm = $_POST['passwordConfirm'];

    $image = $_FILES['image']['name'];
    $tmp_image = $_FILES['image']['tmp_name'];
    $imageSize = $_FILES['image']['size'];

    $conditions = isset($_POST['conditions']);

    $date = date("F, d Y");


    if(strlen($firstName) < 3)
    {
        $error = "First name is too short";
    }

    else if(strlen($lastName) < 3)
    {
        $error = "Last name is too short";
    }
    else if(strlen($userid) > 8)
    {
        $error = "You need a longer username";
    }
    else if(!filter_var($email, FILTER_VALIDATE_EMAIL))
    {
        $error = "Please enter valid email address";
    }
    else if(email_exists($email, $con))
    {
        $error = "Someone is already registered with this email";
    }
    else if(strlen($password) < 8)
    {
        $error = "Password must be greater than 8 characters";
    }
    else if($password !== $passwordConfirm)
    {
        $error = "Password does not match";
    }
    else if($image == "")
    {
        $error = "Please upload your image";
    }
    else if($imageSize > 1048576)
    {
        $error = "Image size must be less than 1 mb";
    }           
    else if(!$conditions)
    {
        $error = "You must be agree with the terms and conditions";
    }
    else
    {   
            $password = password_hash($password, PASSWORD_DEFAULT);

            $imageExt = explode(".", $image);
            $imageExtension = $imageExt[1];

            if($imageExtension == "PNG" || $imageExtension == "png" || $imageExtension == "JPG" || $imageExtension == "jpg")
            {
                $image = rand(0, 100000).rand(0, 100000).rand(0, 100000).time().".".$imageExtension;

                $insertQuery = "INSERT INTO users(firstName, lastName, userid, email, password, image) VALUES ('$firstName','$lastName','$userid','$email','$password','$image')";
                if(mysqli_query($con, $insertQuery))
                {
                    if(move_uploaded_file($tmp_image,"images/$image"))
                    {
                        $error = "You are successfully registered";
                    }
                    else
                    {
                        $error = "Image is not uploaded";
                    }
                }
            }
            else
            {
                $error = "File must be an image";
            }
    }

}

if(logged_in())
{
    header("location:profile.php");
    exit();
}

$error = "";

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

    $email = mysqli_real_escape_string($con, $_POST['email']);
    $password = mysqli_real_escape_string($con, $_POST['password']);
    $checkBox = isset($_POST['keep']);

    if(email_exists($email,$con))
    {
        $result = mysqli_query($con, "SELECT password FROM users WHERE email='$email'");
        $retrievepassword = mysqli_fetch_assoc($result);

        if(!password_verify($password, $retrievepassword['password']))
        {
            $error = "Password is incorrect";
        }
        else
        {
            $_SESSION['email'] = $email;

            if($checkBox == "on")
            {
                setcookie("email",$email, time()+3600);
            }

            header("location: profile.php");
        }


    }
    else
    {
        $error = "Email Does not exists";
    }




}

<?php
session_start();
$_SESSION['FirstName'] = firstName; //The firstname the user typed in
$_SESSION['LastName'] = lastName; //The lastname the user typed in
?>