PHP Include And Variable Issue

I’m storing a value into a variable I’ve created called $mov_sub. I’m using the Exec-PHP plugin to put PHP into posts. This is the PHP I’ve got in my post:

<?php mov_sub = "123"; ?>

Then I’ve got a file included using PHP in my header.php file. The source code for the included file is along the lines of:

<?php

if(is_single( array( 1079, 1088 ) ) )

if(isset($mov_sub)) {
	$sid = "&sid=$mov_sub";
}
else {
	$sid = "&sid=";
}
	echo "$sid";
}
?>

It’s only echoing &sid, not &sid=123 like it should be doing. I tested this outside of WordPress and it works fine. I thought maybe it wasn’t working because WordPress uses a function to create the post content called the_content() and I think functions only allow local variables unless you declare a global variable. So I edited the post-template.php file in wp-includes where the_content() function is located and added:

global $mov_sub;

but that didn’t work either. I’m really new to PHP and have been struggling with this for a few hours now. Does anyone know why it’s not working?