PHP include var from file in a file that is then included in other files

Question

I have a blog structured with several different php files (header, footer, main, etc.). I want to create a PHP file called adv.php that contains a var that needs to be retrieved in all the other PHP files. I would like to include the adv.php file ONLY in the header.php, and make sure that the var still works in footer.php, main.php, and so on.

I tried and even with global vars, this doesn't seem to work. How can I fix this? Again, I would like to only include the adv.php file once in header.php and not in every single php file.

EDIT

Here is a simplified version of the code with only relevant parts:

adv.php

<?php $ad300_top_right_index = "RTB"; ?>

header.php

<?php include_once('adv.php');  ?>

main.php

<?php if  ( $ad300_top_right_index == "RTB" ) { ?>

show code here

<?php } else { ?>

show some other code here

<?php } ?>

About the inclusions: it's a Wordpress template, where the resulting page will include header.php and main.php, and header.php includes adv.php. Of course adv.php gets included before I try to use the var in main.php.

Answer 1

You haven't shown any code, but there is NO reason for a var included in one file to not be visible in the other:

a.php

<?php
$x = 7;

b.php

<?php
echo $x, "\n";

z.php

<?php
echo "#1 - startup\n";
echo $x, "\n";

echo "#2 - including a\n";
include('a.php');

echo "#3 - included\n";
echo $x, "\n";

echo "#4 - including b\n";
include('b.php');

echo "#5 - included\n";

Output of running z.php:

#1 - startup
PHP Notice:  Undefined variable: x in /home/marc/z.php on line 3

#2 - including a
#3 - included
7
#4 - including b
7
#5 - included

Note that 7 is output as expected.

If you're not getting your var, then there's something OTHER than php causing the problem, eg not understanding scoping rules.