function login($email, $password, $mysqli) {
    // Using prepared statements means that SQL injection is not possible.
    if ($stmt = $mysqli->prepare("SELECT id, username, password, salt
        FROM members
       WHERE email = ?
        LIMIT 1")) {
        $stmt->bind_param('s', $email);  // Bind "$email" to parameter.
        $stmt->execute();    // Execute the prepared query.
        $stmt->store_result();
 
        // get variables from result.
        $stmt->bind_result($user_id, $username, $db_password, $salt);
        $stmt->fetch();
 
        // hash the password with the unique salt.
        $password = hash('sha512', $password . $salt);
        if ($stmt->num_rows == 1) {
            // If the user exists we check if the account is locked
            // from too many login attempts
 
            if (checkbrute($user_id, $mysqli) == true) {
                // Account is locked
                // Send an email to user saying their account is locked
                return false;
            } else {
                // Check if the password in the database matches
                // the password the user submitted.
                if ($db_password == $password) {
                    // Password is correct!
                    // Get the user-agent string of the user.
                    $user_browser = $_SERVER['HTTP_USER_AGENT'];
                    // XSS protection as we might print this value
                    $user_id = preg_replace("/[^0-9]+/", "", $user_id);
                    $_SESSION['user_id'] = $user_id;
                    // XSS protection as we might print this value
                    $username = preg_replace("/[^a-zA-Z0-9_\-]+/",
                                                                "",
                                                                $username);
                    $_SESSION['username'] = $username;
                    $_SESSION['login_string'] = hash('sha512',
                              $password . $user_browser);
                    // Login successful.
                    return true;
                } else {
                    // Password is not correct
                    // We record this attempt in the database
                    $now = time();
                    $mysqli->query("INSERT INTO login_attempts(user_id, time)
                                    VALUES ('$user_id', '$now')");
                    return false;
                }
            }
        } else {
            // No user exists.
            return false;
        }
    }
}

<?php
 
include_once 'includes/db_connect.php';
include_once 'includes/functions.php';
 
sec_session_start();
 
$connectedUserID = $_SESSION['userID']; 
 
$stmt = $mysqli->prepare("SELECT rc_usuario.*, rc_referidos.*
	FROM rc_referidos
	INNER JOIN rc_usuario ON rc_usuario.idUsuario = rc_referidos.idReferido
	WHERE rc_referidos.idRedComercio2 = '$connectedUserID'");
 
$stmt->execute();
$stmt->store_result();
 
while ($stmt->fetch()) {
	$stmt['idUsuario'];
	$stmt['nombre'];
	$stmt['apellidos'];
	$stmt['status'];

$stmt->execute();

$stmt['idUsuario'];

echo'<li><a>'.$stmt['idUsuario'].'</a></li>';