Multiple Insert in different tables with insert new AI - php

Question

I have a PHP-script in which I can register news users.
Of course I want to insert things like username, password etc. (these values come from the user) but on the other side I want to insert his user_id (new user!) into another table.
My SQL
$sql = "
BEGIN;
INSERT INTO users (username, password, auth_lvl, realname, usercolor) VALUES ('$username', '$password', '$auth_lvl', '$realname', '$usercolor');
INSERT INTO users_startmodules (user_id, startmodule, enabled) VALUES ('(SELECT MAX(user_id)+1 FROM users)', 'newsmodule', '1');
COMMIT;
";
How can I solve this problem and is BEGIN; ... COMIT; the right choice when I want to get the query canceled if just one thing didn't work, because I don't want to have just a few entries in the worst case.

Answer 1

I suggest to change the type of user_id in users table to auto_increment with something like this:
alter table users alter column user_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY
Now, Assuming that you have escaped the user input data:
$query="INSERT INTO users (username, password, auth_lvl, realname, usercolor) VALUES ('$username', '$password', '$auth_lvl', '$realname', '$usercolor');"
and run that:
$mysqli->query($query);
then get the userid with mysql_insert_id() function of MySQL or alternatively in PHP:
$userid=$mysqli->insert_id;
and:
$query="INSERT INTO users_startmodules (user_id, startmodule, enabled) VALUES ('$userid', 'newsmodule', '1');";
$mysqli->query($query);

Answer 2

This is too long for a comment.
To get the id that was just inserted, use last_insert_id(). This function is documented here.
Next, the answer to your question is to start a transaction. I would give you the syntax, but it is better for you to read the documentation on transactions before you start using them.
The key idea is that a transaction groups data modifications together. Either all take place or none do. Along the way, you can use commit or rollback, depending on whether or not you want to keep the changes or keep none of the changes.