PHP to mySQL - duplicating entry when finding insert ID - php

Question

I'm having an issue with my PHP form submission to MySQL - it's a multi_query insert that is supposed to feed two tables. Everything was working fine, but I recently set up a foreign key relationship and something in the process of getting the mysqli_insert_id for the entry to the parent table causes the first entry to be entered twice in that table - to use the table names from my code below, the master table entry is being inserted twice, and the local table entry is inserted once with the $master_id of the first master entry. I really only have a functional understanding of PHP - can someone explain why this is happening and how I can fix it so that the entry is inserted only once into each table?
$conn = new mysqli($servername, $username, $password, $dbname);
if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);
}
$date = $_POST['date'];
$time = $_POST['time'];
$latitude = $_POST['latitude'];
$longitude = $_POST['longitude'];
$accuracy = $_POST['accuracy'];
$species = $_POST['species'];
$sql = "INSERT INTO master (date, time, latitude, longitude, accuracy, species, source)
VALUES ('$date', '$time', '$latitude', '$longitude', '$accuracy', '$species', 'source A');";
mysqli_query($conn, $sql);
$master_id = mysqli_insert_id($conn);
$sql .= "INSERT INTO local (Master_ID, date, time, latitude, longitude, accuracy, species)
VALUES ('$master_id', '$date', '$time', '$latitude', '$longitude', '$accuracy', '$species');";
if ($conn->multi_query($sql) === TRUE) {
$conn->close();
header("Location:http://d-bird.org/thank%20you.html");
}
else {
echo "Error: " . $sql . "<br>" . $conn->error;
}

Answer 1

$sql .= "INSERT INTO local (Master_ID, date, time, latitude, longitude, accuracy, species)
VALUES ('$master_id', '$date', '$time', '$latitude', '$longitude', '$accuracy', '$species');";
should be
$sql = "INSERT INTO local (Master_ID, date, time, latitude, longitude, accuracy, species)
VALUES ('$master_id', '$date', '$time', '$latitude', '$longitude', '$accuracy', '$species');";
without the dot concatenation for $sql

Answer 2

You are adding the second insert to the first.
changing
$sql .= "INSERT INTO local (Master_ID, date, time, ...
to
$sql = "INSERT INTO local (Master_ID, date, time, ...
should fix your issue.