security vulnerable php code (old)

security_vulnerable_php_code.php

<?php
// Code with the security issue
// Connect to the database
$mysqli = new mysqli ("localhost", "username", "password", "database");
// Check for errors
if ($mysqli->connect_error) {
    die("Connection failed: ". $mysqli->connect_error);}
 // Get the user input from a GET request
 $user_id = $_GET['user_id'];

 // SQL query
 $sql = "SELECT username, email FROM users WHERE id = $user_id";
 $result = $mysqli->query($sql);

 // Fetch results
 if ($result->num_rows > 0) {
     while ($row = $result->fetch_assoc()) {
        echo "Username: " . $row["username"]. " - Email: " . $row["email"] . "<br> ";
    }
 } else {
    echo "0 results";
 }

 $mysqli->close();