php输出代码而不是执行

Question

我对php很新，但是必须通过php将数据库链接到html作为uni课程的一部分，所以我试图在我们创建的网站上实现注册和登录功能。据我所知，这个代码应该可以工作，但每当我尝试通过html页面运行它时，它会发布代码本身而不是运行。这是登录和注册的情况。我做错了什么？

//registration
<?php 
    define('DB_HOST', 'xxx'); 
    define('DB_NAME', 'xxx'); 
    define('DB_USER','xxx'); 
    define('DB_PASSWORD','xxx'); 

    $con = mysql_connect(DB_HOST,DB_USER,DB_PASSWORD) or die("Failed to connect to MySQL: " . mysql_error());

    $db = mysql_select_db(DB_NAME,$con) or die("Failed to connect to MySQL Database: " . mysql_error()); 

function NewUser() { 
    $Name = $_POST['Name']; 
    $Username = $_POST['Username']; 
    $Email = $_POST['Email']; 
    $Password = $_POST['Password']; 

    $query = "INSERT INTO userDetails (Username,Email,Name,Password) VALUES  ('$Username','$Email','$Name','$Password')"; 

    $data = mysql_query($query) or die(mysql_error());

    if($data) { 
        echo "YOUR REGISTRATION IS COMPLETED...";
    }
}

function SignUp()    { 
    if(!empty($_POST['Username'])) { 
        $query = mysql_query("SELECT * FROM userDetails WHERE Username =   '$_POST[Username]' AND Password = '$_POST[Password]'") or die(mysql_error()); 

        if(!$row = mysql_fetch_array($query) or die(mysql_error())) {
            newuser(); 
        } else { 
            echo "YOU ARE ALREADY A REGISTERED USER..."; 
        } 
    } 
} 

if(isset($_POST['submit'])) { 
    SignUp();
}