accessing a $_SESSION variable is failing

I have a little code sitting in a php file that is in the assets directory. I know that is not normal, but this is where the vendor keeps it. When that code runs the line below is executed. 

$campaign=$this->$_SESSION['campaign'];

However,  $campaign yields null (as seen in the debugger). But when I look at it in the debugger, $_SESSION[campaign'] has the string "Apple" in it. How can I get the value of the $_SESSION into the variable $campaign?

proof that an old dog can learn new tricks