PHP 5 - Variable scope across include files with no classes or functions

If none of these files have functions or classes then $prev and $next are in the global scope and should be seen by all your include files and you shouldn't need to use the global keyword.

It sounds like the order of your includes may be a bit wrong.

Update:

If I understand correctly you have something like this:

header.php:

<?php
echo "In header.php\n";

require_once("navigation.php");
require_once("backnext.php");

echo "Also seen in header.php:\n";
echo "prev=$prev\n";
echo "next=$next\n";
?>

navigation.php:

<?php
echo "In navigation.php\n";
$prev = "Hello World#1";
$next = "Hello World#2";
echo "Exiting navigation.php\n";
?>

backnext.php:

<?php
echo "In backnext.php\n";
echo "prev=$prev\n";
echo "next=$next\n";
echo "Exiting backnext.php\n";
?>

If I run this I get:

In header.php
In navigation.php
Exiting navigation.php
In backnext.php
prev=Hello World#1
next=Hello World#2
Exiting backnext.php
Also seen in header.ph
prev=Hello World#1
next=Hello World#2

backnext.php is able to see both $prev and $next.

I think you probably have a logic error somewhere where the contents of the variables are getting wiped, or like @Kev pointed out the flow of execution is wrong. Here is some test code:

File: test.php

<?php
    $test = "Hi!";
    require_once 'test2.php';
    require_once 'test3.php';
?>

File: test2.php

<?php
    echo("Test 2: " . $test . "<br/>");
?>

File: test3.php

<?php
    echo("Test 3: " . $test . "<br/>");
?>

This produces the output:

Test 2: Hi!
Test 3: Hi!

Which proves that the variable $test is globally scoped, and should be available to any script after it is defined.

PS- Don't rely on users of SO to provide your reference material. Go straight to the source: Variable Scope - PHP Manual First paragraph on that page reads:

The scope of a variable is the context within which it is defined. For the most part all PHP variables only have a single scope. This single scope spans included and required files as well.

Edit

Try this in header.php and see what happens:

<?php
    include 'navigation.php';
    echo($previous . " ; " . $next . "<br/>");
    include 'backnext.php';
    echo($previous . " ; " . $next . "<br/>");
?>

If you don't get the same output both times, then there's a problem in backnext.php where the variables are getting wiped. If it does produce the same output, then move the echoes inside backnext.php at the very top and very bottom. Logically its not actually moving them, but you can keep moving them closer and closer together until you find where the issue is.