$error = array();
$error[] = "each time false is returned print some message here";

        if (count($error) === 0)
    {
      header('location: ./login.php');

<h3>Please enter username and password to sign up</h3>
<form action="./signup.php" method="post">
Username: <input type="text" name="username" placeholder="User name"><br>
Password: <input type="password" name="password" placeholder="Password"><br>
<input type="submit" value="Sign up">
</form>