PHP/Database, add MD5 and check

Question

Well, I use the following code to p/ make a check if the user exists and log in:

<?PHP
include('config.php');
include('menu.php');
# Validar os dados do usuário

function anti_sql_injection($string)
{
    include('config.php');
    $string = stripslashes($string);
    $string = strip_tags($string);
    $string = mysqli_real_escape_string($conexao,$string);
    return $string;
}

   $sql = mysqli_query($conexao,"select * from tb_usuario where 
   email_usu='".anti_sql_injection($_POST['email_usu'])."' and 
   senha_usu='".anti_sql_injection($_POST['senha_usu'])."' limit 1") or die("Erro");
$linhas = mysqli_num_rows($sql);
if($linhas == '')
    {
        ?>
        <div class="msg2 padding20">Usuário não encontrado ou usuário e senha inválidos.</div>
        <?PHP
    }
else
    {
        while($dados=mysqli_fetch_assoc($sql))
            {
                session_start();
                $_SESSION['nome_usu_sessao'] = $dados['nome_usu'];
                header("Location: conteudo.php");
            }
    }
?>

As you can see, the code has a protection against Sqlinjection, however I would like to add MD5 encryption or check the password with MD5, I do not know the right term p/ this, but in case "read the MD5".