register.php

<?php

$conn = new mysqli("localhost", "example", "password", "yourDB");

// Check connection

if ($conn->connect_error) {

    die("Connection failed: " . $conn->connect_error);
}

$name=$_POST["username"];

if(isset($_POST["username"]))
{	
	$sql ="INSERT INTO User (username) VALUES('$name')";
			$result=$conn->query($sql);
			$id = mysqli_insert_id($conn);
		echo $id;
}
else
{
	echo 'connection error';
}

?>