mysql insert in two tables

I am trying to make a register form where you can register your organisation and account at the same time.
But at the moment I do not know how to insert the data correctly.
Because if you want to insert the user in the table you do not know the organisation_id yet...

I have tried using mysqli_insert_id($conn) to get the last inserted id but that is not really practical.

I also came to the problem that if the email of the user already existed that it would still register the organisation.

At this moment I am completely lost...

Here is a sample of my table structure with some dummy data:

organisations table

+-----------------+--------------------+
| organisation_id | organisation_name  |
+-----------------+--------------------+
|               1 | Google             |
|               2 | Facebook           |
+-----------------+--------------------+

users table

+---------+------------------+----------+-----------------+
| user_id |      email       | password | organisation_id |
+---------+------------------+----------+-----------------+
|       1 | test@gmail.com   | *****    |       1         | 
|       2 | test@outlook.com | ******   |       2         |
+---------+------------------+----------+-----------------+

         //check for existing emails for user in database
        $get_email_stmt = $conn->prepare('SELECT `email` FROM `users` WHERE email = ?');
        $get_email_stmt->bind_param('s', $user_email);
        $get_email_stmt->execute();
        $get_email_result = $get_email_stmt->get_result();
        $row = $get_email_result->fetch_assoc();
        //if email does not exists execute query
        if ($row['email'] == $user_email) {
           //give error email exists
           $_SESSION["exists"] = "email";
           header('Location: ../register');
        }

        //check for existing organisation name for user in database
        $get_organisation_stmt = $conn->prepare('SELECT `organisation_name` FROM `organisations` WHERE organisation_name = ?');
        $get_organisation_stmt->bind_param('s', $organisation_name);
        $get_organisation_stmt->execute();
        $get_organisation_result = $get_organisation_stmt->get_result();
        $row = $get_organisation_result->fetch_assoc();
        //if email does not exists execute query
        if ($row['organisation_name'] == $organisation_name) {
           //give error email exists
           $_SESSION["exists"] = "organisation";
           header('Location: ../register');
        }

        //insert organisation
        $post_organisation_stmt = $conn->prepare('INSERT INTO `organisations` (organisation_name, zipcode, cityname, country, organisation_phonenumber, organisation_email, organisation_type, organisation_url, organisation_vat, agreed_to_avg) 
        VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?);');
        $post_organisation_stmt->bind_param('sssssssssi', $organisation_name, $zipcode, $cityname, $country, $organisation_phonenumber, $organisation_email, $request, $organisation_url, $organisation_vat, $agreed_to_avg);
       
        //execute query
        if ($post_organisation_stmt->execute()) {
            $organistation_id = mysql_insert_id($conn);
            $post_organisation_stmt->close();
            //insert user
            $post_user_stmt = $conn->prepare('INSERT INTO `users` (firstname, lastname, email, phonenumber, organisation_id, hpassword, permission, agreed_to_avg) 
            VALUES (?, ?, ?, ?, ?, ?, ?, ?);');
            $post_user_stmt->bind_param('sssssssi', $firstname, $lastname, $user_email, $user_phonenumber, $organisation_id, $hPassword, $permission, $agreed_to_avg);
            //execute query
            if ($post_user_stmt->execute()) {
                //succes
                $post_answer_stmt->close();
                $_SESSION["newaccount"] = "success";
                header('Location: ../login');
            } else {
                //error
                echo "Error updating record: " . mysqli_error($conn);
                $post_answer_stmt->close();
                $_SESSION["exists"] = "error";
                header('Location: ../register');
            }
        } 

It seems that the problem you are describing is this:

1. INSERT INTO organisation
2. Get inserted organisation id
3. INSERT INTO user
4. If email already exists, then no user is created but the organisation remains in the database.

This can be resolved by using a transaction, see https://www.php.net/manual/en/mysqli.begin-transaction.php if using mysqli.

When you run both queries in a transaction, then all changes can be rolled back when an exception occurs such as a unique constraint violation for the user email.