The problem is that the & sign is used to seperate arguments in the get statement of your URL (after the question mark). Basically you can not have an & sign in your url as part of an incoming string.
What you need to do is use a %26 in the URL wherever you have an & sign (%26 is the hex ascii number for &).
So you would call
script.php?STRING=SOMETEXTHERE%26THATSIT
And you should get what you are looking for.
Hope this helps.
-Josh