PHP Checking for item in database - php

Question

I'm new to PHP and I am trying to check if a user exists in a MySQL database over a MySQLi connection. The name of the user I'm trying to check for is stored in a variable called $code. What would I insert into the statement below to get it to check for $code?
$stmt = $this->db->prepare("SELECT id_user FROM members WHERE username = ??? ($code)");
Thanks for your help.
Edit: Here is my code:
class RedeemAPI {
private $db;
// Constructor - open DB connection
function __construct() {
$this->db = new mysqli('localhost:3306', 'username', 'password', 'db');
$this->db->autocommit(FALSE);
}
// Destructor - close DB connection
function __destruct() {
$this->db->close();
}
// Main method to redeem a code
function redeem() {
// Check for required parameters
if (isset($_POST["username"])) {
// Put parameters into local variables
$code = $_POST["username"];
// Look up code in database
$id_user= 0;
$stmt = $this->db->prepare('SELECT id_user FROM members WHERE username = ?');
$stmt->bind_param("s", $code);
$stmt->execute();
$stmt->bind_result($id_user);
while ($stmt->fetch()) {
break;
}
$stmt->close();
// Bail if code doesn't exist
if ($id_user <= 0) {
sendResponse(400, 'Invalid code');
return false;
}
// Return username, encoded with JSON
$result = array("username" => $code);
sendResponse(200, json_encode($result));
return true;
}
sendResponse(400, 'Invalid request');
return false;
}
}

Answer 1

See mysqli_stmt::bind_param():
$stmt = $this->db->prepare("SELECT id_user FROM members WHERE username = ?");
$stmt->bindParam('s', $code);
$stmt->exeucte();

Answer 2

Here's a good example of a prepared statement:
if ($stmt = $mysqli->prepare("SELECT id_user FROM members WHERE username = ?")) {
/* bind parameters for markers */
$stmt->bind_param("s", $code);
/* execute query */
$stmt->execute();
/* bind result variables */
$stmt->bind_result($result);
/* fetch values */
while ($stmt->fetch()) {
$result = $result;
}
/* fetch value */
$stmt->fetch();
/* close statement */
$stmt->close();
}