SQLi demonstration.

sqli.php

<?php
session_start();
$conn = mysqli_connect('localhost', 'root', '', 'test');
if(mysqli_connect_error())
{
	echo '<p>Connexion à la base de données échouée : ' . mysqli_connect_error() . '</p>';
	exit;
}
?>

<form method = "post">
	<label><p>Login : <input type = "text" name = "login" /></p></label>
	<label><p>Mot de passe : <input type = "password" name = "mdp" /></p></label>
	<input type = "submit" value = "Connexion" />
</form>

<?php
if(isset($_POST['login'], $_POST['mdp']))
{
	$login = $_POST['login'];
	$mdp = $_POST['mdp'];
	$sql = "SELECT * FROM utilisateurs WHERE login = '$login' AND mdp = '$mdp'";
	echo "<p>Requête SQL : $sql</p>";
	$req = mysqli_query($conn, $sql);
	if($req === false)
		die('<p>Erreur MySQL : ' . mysqli_error($conn) . '</p>');
	if(mysqli_num_rows($req) == 0)
		die('<p>Login ou mot de passe incorrect.</p>');
	echo '<p>Login et mot de passe corrects.</p>';
	$_SESSION['login'] = $login;
}