<form method="POST" /><br/>
    <input name="usuario" type="text" /><br/>
    <input name="senha" type="password" /><br/>
    <input type="submit" value="Enviar" /><br/>
<form>
<?php
$user = "lucas12";
$pass = "exploit";

$u = $_POST['usuario'];
$p = $_POST['senha'];

if($_SERVER['REQUEST_METHOD']=='POST'){
    if($u == $user && $p == $pass){
        echo "Login efetuado com sucesso!";
}else{
        echo "Login ou senha incorreto!";
}
}

?>