MySQL is there a chance of accidental offset of automatic increment - php

Question

If inserting into two mysql tables at the same time is there a chance that another user could insert at the same time making the tables offset?
For example hypothetically: (In php)
$conn = new mysqli($servername, $username, $password, $dbname);
$sql = "INSERT INTO Name (user_id, name, email)
VALUES (DEFAULT, 'User name', 'john#example.com')";
if ($conn->query($sql) === TRUE) {
echo "New record created successfully";
} else {
echo "Error: " . $sql . "<br>" . $conn->error;
}
$sql = "INSERT INTO Location (user_id, addressline1, addressline1, postcode)
VALUES (DEFAULT, '123 street', 'anytown, country', '123456')";
if ($conn->query($sql) === TRUE) {
echo "New record created successfully";
} else {
echo "Error: " . $sql . "<br>" . $conn->error;
}
$conn->close();
If 'John' ran this at the same times as 'Bob' is it possible for john's Name to be given the auto-increment id of 1 and his Location to be given 2 because Bob's Location was given 1 first even though his Name was given 2.
Kinda hard to explain.
I want to know if 1000's if not millions of people are running this
could it occur?
is there some reason MySQL will stop this from
happening?
if not what can/can anything be done to prevent it?

Answer 1

In your scenario, given the code you posted, the thing you're worried about is possible and will definitely happen as you gain more and more users.
Using insert_id from mysqli will solve this issue for you. The autoincremented id from the last successful insert query is stored in here for precisely this reason. Change your second sql query to this.
$sql = "INSERT INTO Location (user_id, addressline1, addressline1, postcode)
VALUES ({$conn->insert_id}, '123 street', 'anytown, country', '123456')";
and its no longer a problem.