如何修复php中的未捕获/致命错误（由于调用未定义的函数）？

Question

在创建网站登录页面时，我试图用PHP代替cgi。以下是我提出的建议：

<?php
 $username = mysqli_connect("localhost", "$_POST['user']", ;  
 $password = $_POST['pass'];

 $username = stripcslashes($username);
 $password = stripcslashes($password);
 $username = mysql_real_escape_string($username,$_POST['user'] );
 $password = mysql_real_escape_string($password,$_POST['pass'] );

 mysqli_connect("localhost", "root", "");
 mysql_select_db("login");

 $result= mysql_query("select * from users where username = '$username' and password = '$password'")
 or die("Failed to query database ".mysql_error());
 $row = mysql_fetch_Array($result);
 if($row=['username'] == $username && $row['password'] == $password)   
      {
      echo "Login success!!!! Welcome ".$row['username'];
      }
 else {
      echo "Failed to login!";
      }

?> 

我在使用XAMPP Apache编译时收到了这些错误。

Notice: Undefined index: user in /opt/lampp/htdocs/process.php on line 2

Notice: Undefined index: pass in /opt/lampp/htdocs/process.php on line 3

Fatal error: Uncaught Error: Call to undefined function mysql_real_escape_string() in /opt/lampp/htdocs/process.php:7 Stack trace: #0 {main} thrown in /opt/lampp/htdocs/process.php on line 7

这是我第一次涉足php，我只是按照YouTube教程进行操作，所以我真的不知道如何解决这个错误。有帮助吗？感谢。