Return function on a external php file - php

Question

I'm trying do develop a simple log in form in order to learn php.
I've created the form and a function which send a query to a mysql database to retrieve username and password (stored in md5 encryption) and control it with data inserted by the user.
The problem is that this function (login() )is in a external file called fun_login.php and when I call it from the page login.php it opens the page fun_login.php and it doesn't come back to login.php (I think because there is some problem with the return value)
In login.php I've included the file fun_login.php with
<?php
include "fun_login.php";
?>
login.php
<div class="container">
<div class="row">
<div class="col-sm-2 col-sm-offset-5">
<form action="fun_login.php" role="form" method="post">
<div class="form-group">
<label for="nome">Username:</label>
<input type="textarea" class="form-control" id="username" name="username">
</div>
<div class="form-group">
<label for="password">Password:</label>
<input type="password" class="form-control" id="password" name="password">
</div>
<button type="submit" class="btn btn-default" id="submitlogin" name="submitlogin">Log In</button>
</form>
<?php
if(isset($_POST['submitlogin'])) {
$control = login($_POST['username'], md5($_POST['password']));
if($control) header("Location: ./index.php");
else header("Location: ./login.php");
}
?>
</div>
</div>
</div>
fun_login.php
<?php
function login($username, $password) {
$myconn = mysql_connect(localhost, root, password);
mysql_select_db('portfolio', $myconn);
$query = "SELECT username,password,admin FROM utenti WHERE username = '" . $username . "' AND password = '" .$password . "';";
$result = mysql_query($query, $myconn)or die('Error, insert query failed');
// conto il numero di occorrenze trovate nel db
$numrows = mysql_num_rows($result);
// se il database è vuoto lo stampo a video
if ($numrows == 0) return false;
// se invece trovo delle occorrenze...
else return true;
}
?>

Answer 1

At the time you're calling
<?php
if(isset($_POST['submitlogin'])) {
$control = login($_POST['username'], md5($_POST['password']));
if($control) header("Location: ./index.php");
else header("Location: ./login.php");
}
?>
in your login.php the headers have been already sent so header('Location:') won't work.
You need to put that before any output (echo or html) gets displayed.
If you look in the error log you'll probably see Warning: Cannot modify header information - headers already sent by
By the looks of it that file is included in a layout or something so your login() call has to be even before that.

Answer 2

What's the error message you are receiving?
use this command at the beginning of your php files:
// Report all PHP errors
error_reporting(E_ALL);
Then you will know which error has occurred, if at all.