Unsetting multiple php pages - php

Question

I have multiple pages in php. What i want is when you click the back button and you're logged out, that certain page will not display. Say, i clicked home, then logged out, display log in form and when back button on the browser is pressed, it will say you're logged out. I tried session destroy, clear cookies but it doesn't seem to work(or I'm doing it wrong). I am new to php and I am still in the process of learning it. Do you have any suggestions? O.O I have my code here:
here is my index.php
<?php
require 'core.inc.php';
if(isset($_SESSION['user'])&&!empty($_SESSION['user'])){
include 'home.php';
$html_block1 = "<script language='javascript'>
alert('Welcome');
</script> ";
echo $html_block1;
}else{
include 'loginform.inc.php';
}
?>
then my loginform
<?php
if (isset($_POST['user']) && isset($_POST['pass']) && !empty($_POST['user'])&& !empty($_POST['pass'])) {
$_username = $_POST['user'];
$_password = $_POST['pass'];
$_passwordhash = md5($_password);
$_file = 'password.txt';
$handle = fopen($_file,'r');
$_pass_from_file =fread($handle,1024);
trim($_pass_from_file);
if($_username == 'il' && ($_passwordhash == $_pass_from_file)){
$_SESSION ['user'] = $_username;
header('Location: index.php');
} else {
$html_block1 = "<script language='javascript'>
alert('Incorrect password');
</script> ";
echo $html_block1;
}
fclose($handle);
} else {
unset($_username);
}
?>
<form class="form" method ="POST" type="loginform.inc.php" >
<input type="text" placeholder="Username" name ="user" >
<input type="password" placeholder="Password" name = "pass" >
<input type = "submit" value = "Login">
<input type = "submit" value = "Change password" name = "change">
</form>
my core
<?php
ob_start();
session_start();
clearstatcache();
?>
home
<?php
echo 'Hi! you\'re home<br>';
echo 'please click link to next page<br>';
echo 'logout???';
?>
other page
<?php
echo "Hi<br>";
echo '<a href = logout.php>logout?</a>'
?>
and my log out
<?php
session_destroy();
header('Location: loginform.inc.php');
ob_flush();
?>
Suggestions or edits are welcome. Thanks a lot!