Prevent-SQLI

Example Scenario :

Vuln :
<?php
$query = "SELECT * FROM users WHERE username = '" . $_POST['username'] . "'";
$query .= " AND password = '" . $_POST['password'] . "'";			 
?>

This code uses PDO with parameterized queries to prevent the SQL injection vulnerability :
<?php
$id = $_GET['id'];
$db_connection = new PDO('mysql:host=localhost;dbname=sql_injection_example', 'dbuser', 'dbpasswd');
//preparing the query
$sql = "SELECT username 
            FROM users
            WHERE id = :id";
$query = $db_connection->prepare($sql);
$query->bindParam(':id', $id);
$query->execute();
//getting the result
$query->setFetchMode(PDO::FETCH_ASSOC);
$result = $query->fetchColumn();
print(htmlentities($result));

use the mysql_real_escape_string() :
$db_connection = mysqli_connect("localhost", "user", "password", "db");
		$username = mysqli_real_escape_string($db_connection, $_POST['username']);
		$password = mysqli_real_escape_string($db_connection, $_POST['password']);
		$query = "SELECT * FROM users WHERE username = '" . $username. "' AND password = '" . $password . "'";