Why is my foreach loop not returning values from $_SESSION variable

The first thing is that session_start(); needs to be at the top of your script so your code can find the SESSION.

http://php.net/manual/en/function.session-start.php

white screen

This is usually a symptom of an error occurring in a script that has error reporting disabled.  Suggest you add error_reporting(E_ALL) to the top of all your PHP scripts and fix the things that cause error, warning and notice messages.  Here is how the session works.  Use session_start() before you access any of the contents of $_SESSION.
https://www.experts-exchange.com/articles/11909/PHP-Sessions-Simpler-Than-You-May-Think.html

Also recommend you treat the MySQL issue as urgent.  Please see the note here.  It's not hard to fix, especially if you use object-oriented MySQLi.  If you don't fix it, your scripts will  begin to fail at run time (perhaps with the white screen of death) and the only thing you won't know is when the failures will being to occur.  I wouldn't want that kind of uncertainty hanging over my head!
https://www.experts-exchange.com/articles/11177/PHP-MySQL-Deprecated-as-of-PHP-5-5-0.html

If you're ever in doubt about what information might be in a PHP variable, you can use var_dump() to visualize the information.   Example: var_dump($_SESSION);

A couple of other things you might want to take a look at...

Don't use fetch_array() functions unless you know exactly why you need them.  Instead choose fetch_object().  It is much  more efficient.  Details here: https://www.experts-exchange.com/articles/12293/AntiPHPatterns-and-AntiPHPractices.html. In the article, look for 27. Overlooking Meaningful Optimization

Avoid the use of die() when what you really want is trigger_error() instead.
https://www.experts-exchange.com/articles/29115/PHP-Error-Handling-Never-Say-die-Again.html

Consider this statement:

$row = mysql_fetch_array($result) or die(mysql_error());

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That's the wrong way to deal with an empty results set.  If your script gets to that line of code, the query was successful!  So there is no reasonable expectation that the script should die, nor that mysql_error() would have a meaningful value.  If $row is empty, it's because the query did not find anything that matched the WHERE clause.  This is a perfectly normal and expected behavior for MySQL databases, and certainly not a reason to terminate the script.

Thanks for comments. However, in my post I state the comment below. So yes, session_start() is at the top of the page.

If I echo my session var then I can see the data that is being passed is in the format wd10,wd11,wd12

OK, but we did not see session_start() in the code snippet, and omitting it is a common mistake.  So please let us see the data.  Please use var_dump($_SESSION) and post the output in the code snippet here.  This should be enough to get the information we need to see.  Please use your browser's view source feature and copy the information from that view, thanks.

<?php
error_reporting(E_ALL);
session_start();
var_dump($_SESSION);

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Here you go ray:

'boxdest2' => string 'WD10,WD11,WD12' (length=14)

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Thanks very much Ray. Exactly what I wanted to see which enabled me to code this correctly. Many thanks

Glad to help, and best of luck with your project, ~Ray