通过PHP创建具有条件的数据库

I am creating a web app to allow database creation though, I want to restrict the creation to one database per user , also i have not yet implemented any security at this point my concern is the logic. I will implement security after I have the correct logic. So please give advice on the logic.

1. I am looking for advice to see how I can improve this script. It is functional and does work at this point.

       //define connection
       $conn = mysqli_connect('localhost', 'root','XXXXX','billing');
   
       //Variables
       $UserEmail = $_SESSION['email'];
       $MysqlUserDataBaseCreate = $_POST['create_database'];
   
       //CheckIfUserExists
       $SeeIfUserExist = "SELECT * FROM database_users WHERE email='$UserEmail';";
       $SqlQueryUserCheck = mysqli_query($conn,$SeeIfUserExist);
       $CheckIfRowDataExist = mysqli_num_rows($SqlQueryUserCheck);
   
   
       //ToCreateDataBaseAndUser
       if($CheckIfRowDataExist < 1){
         $InsertDataBaseIntel ="INSERT INTO database_users(email,check_if_created) VALUES ('$UserEmail','$MysqlUserDataBaseCreate');";
         mysqli_query($conn,$InsertDataBaseIntel);
         $CreateDataBaseForUser ="CREATE DATABASE $MysqlUserDataBaseCreate ;";
         mysqli_query($conn,$CreateDataBaseForUser);
   
       }else{
         echo 'you are restrictd to one database';
       }
   
   
       ?>