PHP what does IF(x & y) mean? [duplicate]

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• Reference — What does this symbol mean in PHP? 17 answers

I am expanding on someone else's code and came across this weird IF statement. The $_SESSION['Auth'] refers to an employees authorization level to determine which pages they can access. $level contains the different levels a page can be restricted to. However, I have no idea what that IF statement with only one "&" is actually doing. I have never seen something like this and can't find anything about it. If I need to post more of the code please request so with a comment.

foreach ($level as $l)
          {
            if ((integer)$l == 255)
            {
              if ($_SESSION['Auth'] == 255)
                $allowed = true;
            } else
              if ($_SESSION['Auth'] & (integer)$l)
                $allowed = true;
          }