html - PHP - Undefined index error: email and password on register form

I'm very new guy on php. I have a website project on my course. I'm stuck on this part and could not move more. Need to help for solving this. I tried to reach email and password like on my code, this error says it's not like this.

I have an error like this:

Notice: Undefined index: email in C:\xampp\htdocs\CONFIG.PHP on line 6
Notice: Undefined index: password in C:\xampp\htdocs\CONFIG.PHP on
line 7 Info could not write on database!

(solved with

if($_SERVER['REQUEST_METHOD'] == 'POST') {
//...

) but now any info does not go to database

config.php:

    ob_start();
    include "dbconnect.php";


    $email=$_POST["email"];
    $password=$_POST["password"];
    $username=$_POST["username"];
    $submit = isset($_POST['submit']) ? $_POST['submit'] : null;
        $sql="select * from users where email = '".$email."' and password = '".$password."' and username = '".$username."'";
        $query=mysql_query($sql);
        $counter=mysql_num_rows($query);
        if ($counter!=0){ 
            echo "This user has been registered before. Please check information.";
        }else{

            $sql2 = "INSERT INTO users(email, password, username) VALUES ('".$email."','".$password."','".$username."'";
            $add = mysql_query($sql2);
             if($submit){ 
                echo "
You have registered successfully!
                Please click. ";
                header("Location:regedIndex.html");
             }else{ 
                echo "Info could not write on database!"; 

             } 

            ob_end_flush();
         }
?>

register.html:

    
        

        
        
        
    
    
    

            

                

                    
                
            


            
        

        

            

            
Register

                

                    
                    
                    
                    
                
        
        














        

            


                

                    

                         
                         
                         
                    
                
            
        

Answer

Add if($_SERVER['REQUEST_METHOD'] == 'POST') to check if the form has been submitted.

e.g.

  if($_SERVER['REQUEST_METHOD'] == 'POST') {
    ob_start();

    include "dbconnect.php";

    $email=$_POST["email"];
    $password=$_POST["password"];
    $username=$_POST["username"];
    $submit = isset($_POST['submit']) ? $_POST['submit'] : null;
    $sql="select * from users where email = '".$email."' and password = '".$password."' and username = '".$username."'";
    $query=mysql_query($sql);
    $counter=mysql_num_rows($query);
    if ($counter!=0){ 

        echo "This user has been registered before. Please check information.";
    }else{
        $sql2 = "INSERT INTO users(email, password, username) VALUES ('".$email."','".$password."','".$username."'";
        $add = mysql_query($sql2);
         if($submit){ 
            echo "
You have registered successfully!
            Please click. ";
            header("Location:regedIndex.html");
         }else{ 
            echo "Info could not write on database!"; 


         } 
        ob_end_flush();
    }
  }
?>