<?php

// connecting to the db
$mysqli = new mysqli(HOST, USERNAME, PASSWORD, DATABASE);

$id = 2;

$stmt = $mysqli -> prepare('SELECT email FROM users WHERE id = ? LIMIT 1');
$stmt -> bind_param('i', $id);
$stmt -> execute();
$stmt -> store_result();
$stmt -> bind_result($email);
$stmt -> fetch();

echo $email;