index.php

<?php
ob_start();
session_start();
error_reporting(0);
 require('conexao.php');

 if(isset($_POST['acao']) && $_POST['acao'] == 'logar'){
 	$login = strip_tags(filter_input(INPUT_POST, 'login'));
 	$senha = strip_tags(filter_input(INPUT_POST, 'senha'));

 	if($login == '' || $senha == ''){
 		echo 'Preencha todos os campos';
 	}else{
 		//$sql = mysql_query("SELECT * FROM usuarios WHERE login = '$login' AND senha = '$senha'");
         $sql = "SELECT * FROM usuarios WHERE login ='$login' AND senha ='$senha'";
         $result = mysql_query($sql);
         $row = mysql_fetch_assoc($result);
         $_SESSION['login'] = $login;
         echo '<pre>';
         print_r($_SESSION['login']);
         echo '</pre>';
         header("Location: painel.php");
    }


 }
?>
<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
</head>
<body>
	<form action="" method="post" enctype="multipart/form-data">
		<label>
		    <span>Login</span>
			<input type="text" name="login">
		</label>
		<label>
			<span>Senha</span>
			<input type="password" name="senha">
		</label>

		<input type="hidden" name="acao" value="logar">
		<input type="submit" value="Logar">
	</form>

</body>
</html>
<?php ob_end_flush();?>