PHP 7.1 throws warning on block of standard code

There is a block of standard code that seems to throw warnings in PHP 7.1 (not PHP 7.0, however) (CMSB 3.10)

(from viewer_functions.php at or about line 835)

if      ($options['categoryFormat'] == 'showall')    { $rootDepthVisible = 'all'; $childDepthVisible = 'all'; $parentVisibility = 'parentBranches'; }
  else if ($options['categoryFormat'] == 'onelevel')   { $rootDepthVisible = '1';   $childDepthVisible = '1';   $parentVisibility = 'parentBranches'; }
  else if ($options['categoryFormat'] == 'twolevel')   { $rootDepthVisible = '2';   $childDepthVisible = '1';   $parentVisibility = 'parentBranches'; }
  else if ($options['categoryFormat'] == 'breadcrumb') { $rootDepthVisible = '0';   $childDepthVisible = '0';   $parentVisibility = 'parentsOnly'; }
  else { die("Unknown category format '" .htmlencode($options['categoryFormat'])."'!"); }

I get invalid numeric warnings on the first line of this block.

Warning: A non-numeric value encountered (then it gives the name of the routine and the line number)

In playing with this, if I change the 'all' value to '999' for the two depth values, it works fine.  I'm guessing something is trying to coerce these values into numeric.

It's not a big deal since I use my own viewer functions and was able to change it anyway without affecting standard code, but I just wanted to point it out.

I don't have a small test case to show this, sadly.  Sorry.